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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2019	May 06, 2019
Document And Entity Information [Abstract]
Entity Registrant Name	ROCKET PHARMACEUTICALS, INC.
Entity Central Index Key	0001281895
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		50,289,437
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1

Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 124,464	$ 111,355
Investments	72,126	94,375
Prepaid expenses and other assets	3,029	3,358
Total current assets	199,619	209,088
Property and equipment, net	7,077	2,027
Goodwill	30,815	30,815
Restricted cash	1,525	1,436
Deposits	455	545
Operating lease right-of-use assets	2,647	0
Investments	0	7,402
Total assets	242,138	251,313
Current liabilities:
Accounts payable and accrued expenses	19,680	15,372
Operating lease liabilities, current	892	0
Total current liabilities	20,572	15,372
Convertible notes, net of unamortized discount	42,297	41,447
Operating lease liabilities, non-current	2,169	0
Other liabilities	23	457
Total liabilities	65,061	57,276
Commitments and contingencies (Note 11)
Shareholders' equity:
Common stock, $0.01 par value, 120,000,000 shares authorized; 45,114,437 and 45,194,736 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively	451	452
Treasury stock, at cost, 0 and 50,000 common shares at March 31, 2019 and December 31, 2018, respectively	0	(668)
Additional paid-in capital	302,039	300,253
Accumulated other comprehensive loss	(89)	(127)
Accumulated deficit	(125,324)	(105,873)
Total shareholders' equity	177,077	194,037
Total liabilities and shareholders' equity	242,138	251,313
Series A Convertible Preferred Shares [Member]
Shareholders' equity:
Preferred shares	0	0
Series B Convertible Preferred Shares [Member]
Shareholders' equity:
Preferred shares	$ 0	$ 0

Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Shareholders' equity:
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	1,000,000	1,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	45,114,437	45,194,736
Common stock, shares outstanding (in shares)	45,114,437	45,194,736
Treasury stock, at cost (in shares)	0	50,000
Series A Convertible Preferred Shares [Member]
Shareholders' equity:
Preferred stock, shares authorized (in shares)	300,000	300,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Series B Convertible Preferred Shares [Member]
Shareholders' equity:
Preferred stock, shares authorized (in shares)	300,000	300,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0

Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Consolidated Statements of Operations (Unaudited) [Abstract]
Revenue	$ 0	$ 0
Operating expenses:
Research and development	15,137	5,743
General and administrative	3,808	8,662
Total operating expenses	18,945	14,405
Loss from operations	(18,945)	(14,405)
Research and development incentives	250	186
Interest expense	(1,604)	(1,427)
Interest and other income net	601	288
Accretion of discount on investments	247	15
Net loss	$ (19,451)	$ (15,343)
Net loss per share attributable to common shareholders - basic and diluted (in dollars per share)	$ (0.43)	$ (0.42)
Weighted-average common shares outstanding - basic and diluted (in shares)	45,122,815	36,137,120

Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Consolidated Statements of Comprehensive Loss (Unaudited) [Abstract]
Net loss	$ (19,451)	$ (15,343)
Other comprehensive loss
Net unrealized gain on investments	38	10
Total comprehensive loss	$ (19,413)	$ (15,333)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Consolidated Statement of Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Preferred Stock [Member] Series A Convertible Preferred Shares [Member]	Preferred Stock [Member] Series B Convertible Preferred Shares [Member]	Common Stock [Member]	Treasury Stock [Member]	Additional Paid in Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Beginning Balance at Dec. 31, 2017	$ 16,060	$ 25,406	$ 68		$ 5,340	$ (31,355)	$ 0	$ 15,519
Beginning Balance (in shares) at Dec. 31, 2017	128,738	126,909	6,795,627
Conversion of convertible preferred shares into common shares (in shares)	128,738	126,909
Conversion of convertible preferred shares into common shares	$ (16,060)	$ (25,406)
Conversion of convertible preferred shares into common shares			$ 194		41,272	0	0	0
Conversion of convertible preferred shares into common shares (in shares)			19,475,788
Exchange of common shares in connection with the Reverse Merger	$ 0	$ 0	$ 68		85,992	0	0	86,060
Exchange of common shares in connection with the Reverse Merger (in shares)	0	0	6,805,608
Issuance of common shares, net of issuance costs of $5.3 million	$ 0	$ 0	$ 63		78,455	0	0	78,518
Issuance of common shares, net of issuance costs of $5.3 million (in shares)	0	0	6,325,000
Issuance of common stock pursuant to settlement of restricted stock units	$ 0	$ 0	$ 1		(1)	0	0	0
Issuance of common stock pursuant to settlement of restricted stock units (in shares)	0	0	1,875
Unrealized comprehensive gain on marketable securities/short term investments	$ 0	$ 0	$ 0		0	0	10	10
Share-based compensation	0	0	0		5,382	0	0	5,382
Net loss	0	0	0		0	(15,343)	0	(15,343)
Ending Balance at Mar. 31, 2018	$ 0	$ 0	$ 394		216,440	(46,698)	10	170,146
Ending Balance (in shares) at Mar. 31, 2018	0	0	39,403,898
Beginning Balance at Dec. 31, 2018			$ 452	$ (668)	300,253	(105,873)	(127)	194,037
Beginning Balance (in shares) at Dec. 31, 2018			45,194,736
Issuance of common stock pursuant to exercise of stock options			$ 0	0	0	0	0	0
Issuance of common stock pursuant to exercise of stock options (in shares)			19,701
Shares repurchase			$ 0	(725)	(2)	0	0	(727)
Shares repurchase (in shares)			0
Retirement of treasury stock			$ (1)	1,393	(1,392)	0	0	0
Retirement of treasury stock (in shares)			100,000
Unrealized comprehensive gain on marketable securities/short term investments			$ 0	0	0	0	38	38
Share-based compensation			0	0	3,180	0	0	3,180
Net loss			0	0	0	(19,451)	0	(19,451)
Ending Balance at Mar. 31, 2019			$ 451	$ 0	$ 302,039	$ (125,324)	$ (89)	$ 177,077
Ending Balance (in shares) at Mar. 31, 2019			45,114,437

Consolidated Statement of Shareholders' Equity (Unaudited) (Parenthetical) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Consolidated Statements of Stockholders' Equity (Unaudited) [Abstract]
Issuance of common shares, issuance costs	$ 5.3

Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating Activities:
Net loss	$ (19,451)	$ (15,343)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion of discount on convertible notes	850	696
Amortization of operating lease right-of-use assets	190	(20)
Depreciation expense	102	83
Share-based compensation expense	3,180	5,382
Amortization of premium on short term investments	0	12
Accretion of discount on investments	(247)	(15)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(307)	(266)
Accounts payable and accrued expenses	(84)	(2,145)
Operating lease liabilities	(210)	0
Net cash used in operating activities	(15,977)	(11,601)
Investing activities:
Cash acquired in connection with the Reverse Merger	0	76,348
Proceeds from maturities of investments	29,935	9,718
Proceeds from sale of property and equipment	0	20
Purchases of property and equipment	(760)	(8)
Net cash provided by investing activities	29,175	86,078
Financing activities:
Proceeds from issuance of common stock, net of issuance costs	0	78,518
Net cash provided by financing activities	0	78,518
Net change in cash, cash equivalents and restricted cash	13,198	152,995
Cash, cash equivalents and restricted cash at beginning of period	112,791	18,349
Cash, cash equivalents and restricted cash at end of period	125,989	171,344
Supplemental disclosure of non-cash financing and investing activities:
Accrued purchases of property and equipment	4,392	0
Retirement of treasury stock	1,395	0
Conversion of convertible preferred stock into common stock	0	41,466
Unrealized gain on investments	38	0
Supplemental cash flow information:
Cash paid for interest	$ 1,495	$ 1,495

Nature of Business

3 Months Ended

Mar. 31, 2019

Nature of Business [Abstract]

Nature of Business

1.	Nature of Business

Rocket Pharmaceuticals, Inc., together with its subsidiaries (collectively, “Rocket” or the “Company”), is a clinical-stage, multi-platform biotechnology company focused on the development of first or best-in-class gene therapies, with direct on-target mechanism of action and clear clinical endpoints, for rare and devastating pediatric diseases. The Company has clinical-stage lentiviral vector (“LVV”) programs currently undergoing clinical testing for Fanconi Anemia (“FA”), a genetic defect in the bone marrow that reduces production of blood cells or promotes the production of faulty blood cells and Leukocyte Adhesion Deficiency-I (“LAD-I” ), a genetic disorder that causes the immune system to malfunction. FA has been in clinical stage testing in the European Union (“EU”) since 2016, and in the United States (“U.S.”), Rocket received investigational new drug (“IND”) clearance for both FA and LAD-I in late 2018. Two additional pre-clinical stage LVV programs include Pyruvate Kinase Deficiency (“PKD”), a rare red blood cell autosomal recessive disorder that results in chronic non-spherocytic hemolytic anemia; and Infantile Malignant Osteopetrosis (“IMO”), a genetic disorder characterized by increased bone density and bone mass secondary to impaired bone resorption. In addition, the Company has an adeno-associated virus (“AAV”), program for Danon disease, a multi-organ lysosomal-associated disorder leading to early death due to heart failure. An IND filing was cleared in Danon disease in early 2019 and human clinical studies are anticipated to commence in the second quarter of 2019. The Company has global commercialization and development rights to all of its product candidates under royalty-bearing license agreements, with the exception of the CRISPR/Cas9 development program for which the Company currently only has development rights.

Risks and Liquidity

3 Months Ended

Mar. 31, 2019

Risks and Liquidity [Abstract]

Risks and Liquidity

2.	Risks and Liquidity

The Company has not generated any revenue and has incurred losses since inception. The Company’s operations are subject to certain risks and uncertainties, including, among others, uncertainty of drug candidate development, technological uncertainty, uncertainty regarding patents and proprietary rights, lack of commercial manufacturing experience, a lack of marketing or sales capability or experience, dependency on key personnel, compliance with government regulations and the need to obtain additional financing. Drug candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities.

The Company’s drug candidates are in the development and clinical stage. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies.

The Company’s consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. The Company has experienced negative cash flows from operations and had an accumulated deficit of $125.3 million as of March 31, 2019. As of March 31, 2019, the Company has $196.6 million of cash, cash equivalents and investments.

On April 18, 2019, the Company completed a public offering of 5,175,000 shares of common stock, for net proceeds of $86.0 million. See Note 16 "Subsequent Events" for additional disclosures regarding the public offering. Considering the proceeds from the public offering, Rocket expects such resources would be sufficient to fund its operating expenses and capital expenditure requirements into the first half of 2021.

In the longer term, the future viability of the Company is dependent on its ability to generate cash from operating activities or to raise additional capital to finance its operations. The Company’s failure to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies.

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies [Abstract]

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies

3.	Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies

Basis of Presentation

The accompanying unaudited interim consolidated financial statements should be read in conjunction with the Company’s consolidated financial statements for the year ended December 31, 2018 included in the Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on March 8, 2019. The unaudited interim consolidated financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of March 31, 2019 and the results of its operations and its cash flows for the three months ended March 31, 2019 and 2018. The financial data and other information disclosed in these consolidated notes related to the three months ended March 31, 2019 and 2018 are unaudited. The results for the three months ended March 31, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019 and any other interim periods or any future year or period.

Principles of Consolidation

The consolidated financial statements represent the consolidation of the accounts of the Company and its subsidiaries in conformity with accounting principles generally accepted in the U.S (“US GAAP”). All intercompany accounts have been eliminated in consolidation.

Use of Estimates

The preparation of the consolidated financial statements in conformity with US GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include but are not limited to goodwill impairment, the accrual of research and development expenses, the valuation of equity transactions and share-based awards. Changes in estimates and assumptions are reflected in reported results in the period in which they become known. Actual results could differ from those estimates.

Significant Accounting Policies

The significant accounting policies used in the preparation of these consolidated financial statements for the three months ended March 31, 2019 are consistent with those disclosed in Note 3 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, except as noted below.

Cash, Cash Equivalents and Restricted Cash

Cash, cash equivalents and restricted cash consists of bank deposits, certificates of deposit and money market accounts with financial institutions. Cash equivalents are carried at cost which approximates fair value due to their short-term nature and which the Company believes do not have a material exposure to credit risk. The Company considers all highly liquid investments with maturities of three months or less from the date of purchase to be cash equivalents. The Company’s cash and cash equivalent accounts, at times, may exceed federally insured limits. The Company has not experienced any losses in such accounts.

Restricted cash consists of deposits collateralizing letters of credit issued by a bank in connection with the Company’s operating leases (See Note 11) and a deposit collateralizing a letter of credit issued by a bank supporting the Company’s Corporate Credit Card. Cash, cash equivalents and restricted cash consist of the following:

	March 31, 2019		December 31, 2018

Cash and cash equivalents	$	124,464	$	111,355
Restricted cash		1,525		1,436
	$	125,989	$	112,791

Leases

The Company adopted ASU 2016-02, Leases (“ASU 2016-02”), as amended on January 1, 2019, which supersedes the current leasing guidance and upon adoption, requires lessees to recognize right-of-use assets and lease liabilities on the balance sheet for all leases with terms longer than 12 months. Leases are classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. Upon the adoption of the guidance, operating leases are capitalized on the balance sheet at the present value of lease payments. The balance sheet amount recorded for existing leases at the date of adoption of ASU 2016-02 was calculated using the applicable incremental borrowing rate at the date of adoption.

The Company adopted ASU 2016-02, including several practical expedients on January 1, 2019. The Company elected the available package of practical expedients which allows the Company to not reassess previous accounting conclusions around whether arrangements are or contain leases, the classification of leases, and the treatment of initial direct costs. The Company also made an accounting policy election to utilize the short-term lease exemption, whereby leases with a term of 12 months or less will not follow the recognition and measurement requirements of the new standard. Upon adoption, the Company recognized total right-of-use assets of $2.6 million, with corresponding liabilities of $3.1 million on the consolidated balance sheets, including the reclassification of $0.5 million from deferred rent to right-of-use assets.

See Note 11 “Commitments and Contingencies” for additional disclosures in accordance with the new lease standard.

Fair Value of Financial Instruments

3 Months Ended

Mar. 31, 2019

Fair Value of Financial Instruments [Abstract]

Fair Value of Financial Instruments

4.	Fair Value of Financial Instruments

Items measured at fair value on a recurring basis are the Company’s investments. The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy:

	Fair Value Measurements as of March 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market mutual funds (included in cash and cash equivalents)	$	61,125	$	-	$	-	$	61,125

United States Treasury securities		72,126		-		-		72,126
Investments		72,126		-		-		72,126

	$	133,251	$	-	$	-	$	133,251

				Fair Value Measurements as of December 31, 2018 Using:
	Level 1			Level 2			Level 3			Total
Assets:
Money market mutual funds (included in cash and cash equivalents)		$	30,552		$	-		$	-		$	30,552
United States Treasury securities			101,777			-			-			101,777
Investments			101,777			-			-			101,777
		$	132,329		$	-		$	-		$	132,329

The Company classifies its money market mutual funds and U.S. Treasury securities as Level 1 assets under the fair value hierarchy, as these assets have been valued using quoted market prices in active markets without any valuation adjustment.

Property and Equipment

3 Months Ended

Mar. 31, 2019

Property and Equipment [Abstract]

Property and Equipment

5.	Property and Equipment

The Company’s property and equipment consisted of the following:

	March 31, 2019			December 31, 2018
Laboratory equipment	$	1,556		$	1,556
Leasehold improvements		282			29
Furniture and fixtures		273			273
Computer equipment		179			179
Construction in progress		5,368			469
		7,658			2,506
Less: accumulated depreciation		(581	)		(479	)
	$	7,077		$	2,027

Construction in progress comprises costs associated with the one of the facilities under an operating lease. See Note 11. During the three months ended March 31, 2019 and 2018, the Company recognized $102 and $83 of depreciation expense, respectively.

Accounts Payable and Accrued Expenses

3 Months Ended

Mar. 31, 2019

Accounts Payable and Accrued Expenses [Abstract]

Accounts Payable and Accrued Expenses

6.	Accounts Payable and Accrued Expenses

At March 31, 2019 and December 31, 2018, the Company’s accounts payable and accrued expenses consisted of the following:

	March 31, 2019		December 31, 2018
Research and development	$	12,573	$	10,414
Construction in progress		4,475		-
Government grant payable		541		534
Professional fees		536		690
Accrued interest		493		1,241
Bonus		471		1,774
Other		374		589
Accrued vacation		217		123
Severance and benefits		-		7
	$	19,680	$	15,372

Debt

3 Months Ended

Mar. 31, 2019

Debt [Abstract]

Debt

On January 4, 2018, in connection with the Reverse Merger, the Company assumed the obligations of Inotek Pharmaceuticals Corporation (“Inotek”) under its outstanding convertible notes, with an aggregate principal value of $52.0 million, (the “2021 Convertible Notes”). The 2021 Convertible Notes were issued in 2016 and mature on August 1, 2021 (the “Maturity Date”). The 2021 Convertible Notes are unsecured, and accrue interest at a rate of 5.75% per annum, interest is payable semi-annually on February 1 and August 1 of each year. Each holder of the 2021 Convertible Notes (“Holder”) has the option until the close of business on the second business day immediately preceding the Maturity Date to convert all, or any portion, of the 2021 Convertible Notes held by it at a conversion rate of 31.1876 shares of the Company’s common stock per $1.00 principal amount of 2021 Convertible Notes (the “Conversion Rate”) which is $32.08 per share. The Conversion Rate is subject to adjustment from time to time upon the occurrence of certain events, including the issuance of stock dividends and payment of cash dividends.

The Company, at its option, may redeem for cash all or any portion of the 2021 Convertible Notes if the last reported sale price of a share of the Company’s common stock is equal to or greater than 200% of the conversion price for the 2021 Convertible Notes then in effect for at least 20 trading days (whether or not consecutive) during any 30 consecutive trading day period (including the last trading day of such period) ending within the five trading days immediately preceding the date on which the Company provides notice of redemption, at a redemption price equal to 100% of the principal amount of the 2021 Convertible Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date.

The 2021 Convertible Notes are considered a hybrid financial instrument consisting of a fixed interest rate “host” and various embedded features that required evaluation as potential embedded derivatives under FASB ASC 815, Derivatives and Hedging (“ASC 815”). Based on the nature of the host instrument and the embedded features, management concluded that none of the conversion, put and redemption features required bifurcation and separate accounting from the host instrument. The Company determined that the Additional Interest was an embedded derivative that contains non-credit related events of default. As a result, the Additional Interest feature required bifurcation and separate accounting under ASC 815. Based on the amount of Additional Interest that would be owed and the likelihood of occurrence, Rocket estimated the fair value of the Additional Interest feature to be insignificant upon issuance and as of March 31, 2019 and December 31, 2018.

The Company recorded the 2021 Convertible Notes at their fair value of $38,388 on January 4, 2018, the date of the Reverse-Merger. The difference between the fair value of the 2021 Convertible Notes and the principal value represents a discount on the notes that is being accreted to interest expense over the remaining term using the effective interest method. As of March 31, 2019, the stated interest rate was 5.75%, and the effective interest rate was 15.3%.

The table below summarizes the carrying value of the 2021 Convertible Notes as of March 31, 2019:

Principal amount	$	52,000
Discount		(9,703	)
Carrying value as of March 31, 2019	$	42,297

Accretion of the 2021 Convertible Notes discount was $0.9 million and $0.7 million for the three months ended March 31, 2019 and 2018, respectively.

Share Based Compensation

3 Months Ended

Mar. 31, 2019

Share Based Compensation [Abstract]

Share Based Compensation

8.	Share Based Compensation

Share Option Valuation

The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the share options granted to employees, non-employees and directors for the three months ended March 31, 2019 and the share options granted to employees and directors for the three months ended March 31, 2018 were as follows:

	Three Months Ended March 31,
	2019			2018

Risk-free interest rate		2.61	%		2.57	%
Expected term (in years)		5.77			5.76
Expected volatility		74.60	%		88.60	%
Expected dividend yield		0.00	%		0.00	%
Exercise price	$	14.61		$	17.52
Fair value of common stock	$	14.61		$	17.52

The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the share options granted to non-employees for the three months ended March 31, 2018 were as follows:

	Three Months Ended March 31, 2019

Risk-free interest rate		2.74	%
Expected term (in years)		10.00
Expected volatility		83.79	%
Expected dividend yield		0.00	%
Exercise price	$	18.75
Fair value of common stock	$	18.75

The Company recognizes compensation expense for only the portion of awards that are expected to vest.

The following table summarizes stock option activity for the three months ended March 31, 2019 under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term (Years)		Aggregate Intrinsic Value

Outstanding as of December 31, 2018		8,615,997		$	4.48		7.51	$	94,474
Granted		1,023,366			14.61		9.79
Exercised		(19,701	)		1.92				801
Forfeited		(226,841	)		8.67
Outstanding as of March 31, 2019		9,392,821		$	5.56		7.55	$	114,173

Options vested and exercisable as of March 31, 2019		6,777,244		$	2.17		6.92	$	104,547
Options unvested as of March 31, 2019		2,617,828		$	14.35		9.20	$	9,626

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The intrinsic value of options exercised and exercisable as of March 31, 2019 and 2018 was $114 and $130.

The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2019 and 2018 was $9.60 and $12.84, respectively.

The total fair value of options vested during the three months ended March 31, 2019 and 2018 was $20,493 and $24,724, respectively.

Share-Based Compensation

	Three Months Ended March 31,
	2019		2018

Research and development	$	1,565	$	2,207
General and administrative		1,615		3,175
Total share based compensation expense	$	3,180	$	5,382

As of March 31, 2019, the Company had an aggregate of $24.9 million of unrecognized share-based compensation cost, which is expected to be recognized over the weighted average period of 2.07 years.

Shareholders' Equity

3 Months Ended

Mar. 31, 2019

Shareholders' Equity [Abstract]

Shareholders' Equity

9.	Shareholders’ Equity

On December 27 and 28, 2018, the Company repurchased 100,000 shares of its common stock for aggregate consideration of approximately $1.4 million. The repurchases were made on the Nasdaq Stock Market at prevailing market prices in accordance with SEC Rule 10b-18. 50,000 of the shares repurchased at an average price of $13.36 by the Company settled on December 31, 2018 and the remaining 50,000 shares repurchased at an average price of $14.50 settled on January 2, 2019. As of December 31, 2018, the Company recorded a prepaid expense of $0.7 million related to the 50,000 shares that settled on January 2, 2019 and recorded treasury stock of $0.7 million relating to the 50,000 shares that settled as of December 31, 2018. These shares were subsequently retired in January 2019.

The Company has 14,102 warrants outstanding as of March 31, 2019, convertible into 14,102 shares of common shares at an exercise price of $24.82 per share which expire on June 28, 2023.

Net Loss Per Share

3 Months Ended

Mar. 31, 2019

Net Loss Per Share [Abstract]

Net Loss Per Share

10.	Net Loss Per Share

Basic and diluted net loss per share attributable to common shareholders was calculated as follows:

	Three Months Ended March 31,
	2019			2018

Numerator:
Net loss	$	(19,451	)	$	(15,343	)
Denominator:
Weighted-average common shares outstanding - basic and diluted		45,122,815			36,137,120
Net loss per share- basic and diluted	$	(0.43	)	$	(0.42	)

The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common shareholders for the periods indicated because including them would have had an anti-dilutive effect:

	Three Months Ended March 31,
	2019		2018
Shares issuable upon conversion of the 2021 Convertible Notes		1,620,848		1,620,948
Warrants exercisable for common shares		14,102		14,102
Options to purchase common shares		9,392,821		8,635,089
		11,027,771		10,270,139

Commitments and Contingencies

3 Months Ended

Mar. 31, 2019

Commitments and Contingencies [Abstract]

Commitments and Contingencies

11.	Commitments and Contingencies

Operating Leases

On August 14, 2018, Rocket entered into a lease for approximately 92,000 rentable square feet in Cranbury, New Jersey, for office space, process development, research activities and manufacturing to support the Company’s pipeline (the “NJ Lease Agreement”). The term of the NJ Lease Agreement will commence for 72,000 rentable square feet upon substantial completion of leasehold improvements (the “Commencement Date”), and the remaining 20,000 square feet will commence upon the earlier of the Company’s election to commence the lease of such additional space or thirty months from the Commencement Date. The NJ Lease Agreement has a term of fifteen years from the Commencement Date, with an option to renew for two consecutive five-year renewal terms. Estimated rent payments are $1.2 million per annum, payable in monthly installments, depending upon the nature of the leased space, and subject to annual base rent increases of 3%. The total commitment under the lease is estimated to be approximately $26.5 million over the 15 year term of the lease. The Company delivered a cash security deposit of $0.3 million to the landlord in connection with the NJ Lease Agreement which has been reflected in deposits in the consolidated balance sheets. The Company entered into the lease prior to the building being available for use as the building construction was not complete. The Company has determined it does not control the leased asset prior to the commencement date, but is involved with the design and construction of the space in selecting building designs, general contractors, and funding certain construction costs.

The total restricted cash balance for the Company’s operating leases at March 31, 2019 and December 31, 2018 was $1.0 million and $1.4 million, respectively.

The Company determines if an arrangement is a lease at inception. Operating leases are included in our balance sheet as right-of-use assets from operating leases, current operating lease liabilities and long-term operating lease liabilities. Certain of the Company’s lease agreements contain renewal options; however, the Company does not recognize right-of-use assets or lease liabilities for renewal periods unless it is determined that the Company is reasonably certain of renewing the lease at inception or when a triggering event occurs. As the Company’s leases do not provide an implicit rate, the Company estimated the incremental borrowing rate in calculating the present value of the lease payments. The Company has utilized its incremental borrowing rate based on the long -term borrowing costs of comparable companies in the biotechnology industry. Since the Company elected to account for each lease component and its associated non-lease components as a single combined lease component, all contract consideration was allocated to the combined lease component. Some of the Company’s lease agreements contain rent escalation clauses (including index-based escalations). The Company recognizes the minimum rental expense on a straight-line basis based on the fixed components of a lease arrangement. The Company amortizes this expense over the term of the lease beginning with the date of initial possession, which is the date the Company can enter the leased space and begin to make improvements in preparation for its intended use. Variable lease components represent amounts that are not fixed in nature and are not tied to an index or rate, and are recognized as incurred.

Lease cost	March 31,2019
Operating lease cost	$	251
Total lease cost	$	251

The following table summarizes the maturity of the Company’s lease liabilities on an undiscounted cash flow basis and a reconciliation to the operating lease liabilities recognized on our balance sheet as of March 31, 2019:

Maturity of lease liabilities	March 31,2019
2019 (remaining nine months)	$	818
2020		1,103
2021		894
2022		572
2023		74
Total lease payments	$	3,460
Less: interest		(399	)
Total operating lease liabilities	$	3,061

The following disclosure is provided for periods prior to adoption of ASU 2016-02. Future annual minimum lease payment commitments as of March 31, 2019 were as follows:

2019 (remaining nine months)	$	1,188
2020		1,970
2021		1,898
2022		1,757
2023		1,618
Thereafter		20,144
Total	$	28,575

Leases	March 31, 2019

Operating right-of-use assets	$	2,647
Operating current lease liabilities		892
Operating noncurrent lease liabilities		2,169
Total operating lease liabilities	$	3,061

Other information

Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases		270
Weighted-average remaining lease term - operating leases	3.3 years
Weighted-average discount rate - operating leases		7.77	%

Rent expense was $0.2 million and $0.1 million for the three months ended March 31, 2019 and 2018, respectively.

Litigation

From time to time, the Company may be subject to other various legal proceedings and claims that arise in the ordinary course of its business activities. Although the results of litigation and claims cannot be predicted with certainty, the Company does not believe it is party to any other claim or litigation the outcome of which, if determined adversely to the Company, would individually or in the aggregate be reasonably expected to have a material adverse effect on its business. Regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors.

Indemnification Arrangements

Pursuant to its bylaws and as permitted under Delaware law, the Company has indemnification obligations to directors, officers, employees or agents of the Company or anyone serving in these capacities. The maximum potential amount of future payments the Company could be required to pay is unlimited. The Company has insurance that reduces its monetary exposure and would enable it to recover a portion of any future amounts paid. As a result, the Company believes that the estimated fair value of these indemnification commitments is minimal.

Throughout the normal course of business, the Company has agreements with vendors that provide goods and services required by the Company to run its business. In some instances, vendor agreements include language that requires the Company to indemnify the vendor from certain damages caused by the Company’s use of the vendor’s goods and/or services. The Company has insurance that would allow it to recover a portion of any future amounts that could arise from these indemnifications. As a result, the Company believes that the estimated fair value of these indemnification commitments is minimal.

Agreements Related to Intellectual Property

3 Months Ended

Mar. 31, 2019

12.	Agreements Related to Intellectual Property

The Company has various license and research and collaboration arrangements. The transactions principally resulted in the acquisition of rights to intellectual property which is in the preclinical phase and has not been tested for safety or feasibility. In all cases, the Company did not acquire tangible assets, processes, protocols or operating systems. The Company expenses the acquired intellectual property rights as of the acquisition date on the basis that the cost of intangible assets purchased from others for use in research and development activities, has no alternative future uses.

Strategic Research Collaboration

3 Months Ended

Mar. 31, 2019

Strategic Research Collaboration [Abstract]

Strategic Research Collaboration

13.	Strategic Research Collaboration

On May 16, 2018, Rocket and the Stanford University School of Medicine (“Stanford University”) entered into a strategic collaboration agreement to support the advancement of FA and PKD gene therapy research. Under the terms of the collaboration agreement, Stanford University will serve as the lead clinical trial research center in the U.S. for the planned FA registrational trial and would also be the lead U.S. site for PKD clinical trials. The project will also separately evaluate the potential for non-myeloablative, non-genotoxic antibody-based conditioning regimens as a future development possibility that may be applied across bone marrow-derived disorders. In addition, Rocket agreed to support expansion of Stanford University’s Laboratory for Cell and Gene Therapy (“LCGM”) in order to further enhance the development of Rocket’s internal pipeline. Rocket agreed to contribute up to $3.5 million for the LCGM expansion of which 40% or $1.4 million was due upon execution of the collaboration agreement and the remaining $2.1 million balance is due upon the achievement of certain milestones. In January 2019, the Company and Stanford University signed a Clinical Trial Agreement for the treatment of FA. Upon the signing of the Clinical Trial Agreement, the second milestone of $1.4 million for the LGCM became due and was accrued and expensed in January 2019, when the milestone was met, and paid in April 2019. During the three months ended March 31, 2019, none of the remaining milestones were met with regard to the LCGM.

Related Party Transactions

3 Months Ended

Mar. 31, 2019

Related Party Transactions

14.	Related Party Transactions

During March 2018, the Company entered into a consulting agreement with a member of the Board of Directors for strategic and corporate consulting services to be provided to the Company. The Company incurred expenses of $4 and $30 during the three months ended March 31, 2019 and 2018, respectively relating to services provided under this consulting agreement.

During April 2018, the Company entered into a consulting agreement with a member of the Board of Directors for business development consulting services. Payments for the services under the agreement are $28 per quarter, and the Company may terminate the agreement with 14 days’ notice. The Company incurred expenses of $28 and $0 during the three months ended March 31, 2019 and 2018, relating to services provided under this consulting agreement.

401(k) Savings Plan

3 Months Ended

Mar. 31, 2019

401(k) Savings Plan [Abstract]

401(k) Savings Plan

15.	401(k) Savings Plan

The Company has a defined contribution savings plan (the “Plan”) under Section 401(k) of the Internal Revenue Code of 1986. This Plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Company contributions to the Plan may be made at the discretion of the Company’s board of directors. The Company has elected to match 4% of employee contributions to the Plan, subject to certain limitations. The Company’s matching contribution for the three months ended March 31, 2019 and 2018 was $81 and $30, respectively.

Subsequent Events

3 Months Ended

Mar. 31, 2019

Subsequent Events [Abstract]

Subsequent Events

16.	Subsequent Events

On April 18, 2019, the Company completed a public offering of 5,175,000 shares of common stock, which includes the full exercise of the underwriters’ option to purchase 675,000 additional shares of its common stock, at a public offering price of $17.50 per share. The gross proceeds to Rocket from the public offering were $90.6 million, less $4.6 million of offering costs, commissions, legal and other expenses for net proceeds from the offering of $86.0 million.

On April 30, 2019, the Company announced the California Institute for Regenerative Medicine (“CIRM”) has awarded Rocket a $6.5 million CLIN2 grant award to support the clinical development of gene therapy for LAD-I. Rocket’s IND application for RP-L201 was accepted by the U.S. Food and Drug Administration in November 2018. Proceeds from the grant will help fund clinical trial costs as well as manufactured drug product for Phase I/II patients enrolled in the U.S. clinical site, University of California, Los Angeles Mattel Children’s Hospital, led by PI Donald Kohn, M.D., UCLA Professor of Microbiology, Immunology and Molecular Genetics, Pediatrics (Hematology/Oncology), Molecular and Medical Pharmacology and member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA.

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies [Abstract]

Basis of Presentation

Principles of Consolidation

Use of Estimates

Significant Accounting Policies

Cash, Cash Equivalents and Restricted Cash

	March 31, 2019		December 31, 2018

Cash and cash equivalents	$	124,464	$	111,355
Restricted cash		1,525		1,436
	$	125,989	$	112,791

Leases

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2019

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies [Abstract]

Cash, Cash Equivalents and Restricted Cash

Cash, cash equivalents and restricted cash consist of the following:

	March 31, 2019		December 31, 2018

Cash and cash equivalents	$	124,464	$	111,355
Restricted cash		1,525		1,436
	$	125,989	$	112,791

Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value of Financial Instruments [Abstract]

Fair Value of Financial Instruments Measured on Recurring Basis

The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy:

	Fair Value Measurements as of March 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Money market mutual funds (included in cash and cash equivalents)	$	61,125	$	-	$	-	$	61,125

United States Treasury securities		72,126		-		-		72,126
Investments		72,126		-		-		72,126

	$	133,251	$	-	$	-	$	133,251

				Fair Value Measurements as of December 31, 2018 Using:
	Level 1			Level 2			Level 3			Total
Assets:
Money market mutual funds (included in cash and cash equivalents)		$	30,552		$	-		$	-		$	30,552
United States Treasury securities			101,777			-			-			101,777
Investments			101,777			-			-			101,777
		$	132,329		$	-		$	-		$	132,329

Property and Equipment (Tables)

3 Months Ended

Mar. 31, 2019

Property and Equipment [Abstract]

Property and Equipment

The Company’s property and equipment consisted of the following:

	March 31, 2019			December 31, 2018
Laboratory equipment	$	1,556		$	1,556
Leasehold improvements		282			29
Furniture and fixtures		273			273
Computer equipment		179			179
Construction in progress		5,368			469
		7,658			2,506
Less: accumulated depreciation		(581	)		(479	)
	$	7,077		$	2,027

Accounts Payable and Accrued Expenses (Tables)

3 Months Ended

Mar. 31, 2019

Accounts Payable and Accrued Expenses [Abstract]

Accounts Payable and Accrued Expenses

At March 31, 2019 and December 31, 2018, the Company’s accounts payable and accrued expenses consisted of the following:

	March 31, 2019		December 31, 2018
Research and development	$	12,573	$	10,414
Construction in progress		4,475		-
Government grant payable		541		534
Professional fees		536		690
Accrued interest		493		1,241
Bonus		471		1,774
Other		374		589
Accrued vacation		217		123
Severance and benefits		-		7
	$	19,680	$	15,372

Debt (Tables)

3 Months Ended

Mar. 31, 2019

Debt [Abstract]

Carrying Value of Convertible Notes

The table below summarizes the carrying value of the 2021 Convertible Notes as of March 31, 2019:

Principal amount	$	52,000
Discount		(9,703	)
Carrying value as of March 31, 2019	$	42,297

Share Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Share Based Compensation [Abstract]

Weighted-Average Assumptions for Stock Options

	Three Months Ended March 31,
	2019			2018

Risk-free interest rate		2.61	%		2.57	%
Expected term (in years)		5.77			5.76
Expected volatility		74.60	%		88.60	%
Expected dividend yield		0.00	%		0.00	%
Exercise price	$	14.61		$	17.52
Fair value of common stock	$	14.61		$	17.52

	Three Months Ended March 31, 2019

Risk-free interest rate		2.74	%
Expected term (in years)		10.00
Expected volatility		83.79	%
Expected dividend yield		0.00	%
Exercise price	$	18.75
Fair value of common stock	$	18.75

Stock Option Activity

The following table summarizes stock option activity for the three months ended March 31, 2019 under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term (Years)		Aggregate Intrinsic Value

Outstanding as of December 31, 2018		8,615,997		$	4.48		7.51	$	94,474
Granted		1,023,366			14.61		9.79
Exercised		(19,701	)		1.92				801
Forfeited		(226,841	)		8.67
Outstanding as of March 31, 2019		9,392,821		$	5.56		7.55	$	114,173

Options vested and exercisable as of March 31, 2019		6,777,244		$	2.17		6.92	$	104,547
Options unvested as of March 31, 2019		2,617,828		$	14.35		9.20	$	9,626

Stock-Based Compensation Expense

Share-Based Compensation

	Three Months Ended March 31,
	2019		2018

Research and development	$	1,565	$	2,207
General and administrative		1,615		3,175
Total share based compensation expense	$	3,180	$	5,382

Net Loss Per Share (Tables)

3 Months Ended

Mar. 31, 2019

Net Loss Per Share [Abstract]

Basic and Diluted Net Loss Per Share

Basic and diluted net loss per share attributable to common shareholders was calculated as follows:

	Three Months Ended March 31,
	2019			2018

Numerator:
Net loss	$	(19,451	)	$	(15,343	)
Denominator:
Weighted-average common shares outstanding - basic and diluted		45,122,815			36,137,120
Net loss per share- basic and diluted	$	(0.43	)	$	(0.42	)

Antidilutive Securities Excluded from Computation of Diluted Net Loss Per Share

	Three Months Ended March 31,
	2019		2018
Shares issuable upon conversion of the 2021 Convertible Notes		1,620,848		1,620,948
Warrants exercisable for common shares		14,102		14,102
Options to purchase common shares		9,392,821		8,635,089
		11,027,771		10,270,139

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2019

Commitments and Contingencies [Abstract]

Lease Cost

Variable lease components represent amounts that are not fixed in nature and are not tied to an index or rate, and are recognized as incurred.

Lease cost	March 31,2019
Operating lease cost	$	251
Total lease cost	$	251

Maturities of Lease Liabilities

Maturity of lease liabilities	March 31,2019
2019 (remaining nine months)	$	818
2020		1,103
2021		894
2022		572
2023		74
Total lease payments	$	3,460
Less: interest		(399	)
Total operating lease liabilities	$	3,061

Future Annual Minimum Lease Payment Commitments

Future annual minimum lease payment commitments as of March 31, 2019 were as follows:

2019 (remaining nine months)	$	1,188
2020		1,970
2021		1,898
2022		1,757
2023		1,618
Thereafter		20,144
Total	$	28,575

Balance Sheet Information Related to Operating Lease

Leases	March 31, 2019

Operating right-of-use assets	$	2,647
Operating current lease liabilities		892
Operating noncurrent lease liabilities		2,169
Total operating lease liabilities	$	3,061

Other information

Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases		270
Weighted-average remaining lease term - operating leases	3.3 years
Weighted-average discount rate - operating leases		7.77	%

Nature of Business (Details)	3 Months Ended
Nature of Business (Details)	Mar. 31, 2019 Program
Programs in Clinical Testing [Abstract]
Number of additional programs in preclinical stages targeting other rare genetic diseases	2

Risks and Liquidity (Details) - USD ($) $ in Thousands		3 Months Ended
Risks and Liquidity (Details) - USD ($) $ in Thousands	Apr. 18, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018
Risks and Liquidity [Abstract]
Accumulated deficit			$ (125,324)	$ (105,873)
Cash, cash equivalents and investments			$ 196,600
Common Stock [Member]
Shareholders' Equity Disclosure [Abstract]
Common stock shares issued (in shares)		6,325,000
IPO [Member] \| Common Stock [Member] \| Subsequent Event [Member]
Shareholders' Equity Disclosure [Abstract]
Common stock shares issued (in shares)	5,175,000
Net proceeds, offering amount	$ 86,000

Basis of Presentation, Principles of Consolidation and Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Cash, Cash Equivalents and Restricted Cash [Abstract]
Cash and cash equivalents	$ 124,464	$ 111,355
Restricted cash	1,525	1,436
Total	125,989	112,791	$ 171,344	$ 18,349
Leases [Abstract]
Operating lease, right-of-use assets	2,647	$ 0
Operating lease, liability	3,061
ASU 2016-02 [Member]
Leases [Abstract]
Operating lease, right-of-use assets	2,600
Operating lease, liability	3,100
Reclassification of deferred rent to right-of-use assets	$ 500

Fair Value of Financial Instruments (Details) - Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets [Abstract]
Fair value of financial instruments	$ 133,251	$ 132,329
Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	61,125	30,552
United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	72,126	101,777
Investments [Member]
Assets [Abstract]
Cash and cash equivalents	72,126	101,777
Level 1 [Member]
Assets [Abstract]
Fair value of financial instruments	133,251	132,329
Level 1 [Member] \| Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	61,125	30,552
Level 1 [Member] \| United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	72,126	101,777
Level 1 [Member] \| Investments [Member]
Assets [Abstract]
Cash and cash equivalents	72,126	101,777
Level 2 [Member]
Assets [Abstract]
Fair value of financial instruments	0	0
Level 2 [Member] \| Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Level 2 [Member] \| United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Level 2 [Member] \| Investments [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Level 3 [Member]
Assets [Abstract]
Fair value of financial instruments	0	0
Level 3 [Member] \| Money Market Mutual Funds [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Level 3 [Member] \| United States Treasury Securities [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Level 3 [Member] \| Investments [Member]
Assets [Abstract]
Cash and cash equivalents	$ 0	$ 0

Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Property and Equipment [Abstract]
Property and equipment, Gross	$ 7,658		$ 2,506
Less: accumulated depreciation	(581)		(479)
Property and equipment, net	7,077		2,027
Depreciation expense	102	$ 83
Laboratory Equipment [Member]
Property and Equipment [Abstract]
Property and equipment, Gross	1,556		1,556
Leasehold Improvements [Member]
Property and Equipment [Abstract]
Property and equipment, Gross	282		29
Furniture and Fixtures [Member]
Property and Equipment [Abstract]
Property and equipment, Gross	273		273
Computer Equipment [Member]
Property and Equipment [Abstract]
Property and equipment, Gross	179		179
Construction in Progress [Member]
Property and Equipment [Abstract]
Property and equipment, Gross	$ 5,368		$ 469

Accounts Payable and Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accounts Payable and Accrued Expenses [Abstract]
Research and development	$ 12,573	$ 10,414
Construction in progress	4,475	0
Government grant payable	541	534
Professional fees	536	690
Accrued interest	493	1,241
Bonus	471	1,774
Other	374	589
Accrued vacation	217	123
Severance and benefits	0	7
Total	$ 19,680	$ 15,372

Debt (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jan. 04, 2018
Debt [Abstract]
Accretion of discount on convertible notes	$ 850	$ 696
Carrying value of convertible notes [Abstract]
Carrying value	$ 42,297		$ 41,447
2021 Convertible Notes [Member]
Debt [Abstract]
Debt instrument maturity date	Aug. 01, 2021
Convertible senior notes, interest rate, stated percentage	5.75%
Debt conversion ratio	31.1876
Conversion price (in dollars per share)	$ 32.08
Conversion per principal amount of debt	$ 1
Percentage of common stock conversion price	200.00%
Number of trading days immediately preceding notice of redemption	5 days
Redemption price percentage	100.00%
Effective interest rate	15.30%
Fair value convertible note				$ 38,388
Accretion of discount on convertible notes	$ 850	$ 696
Carrying value of convertible notes [Abstract]
Principal amount	52,000
Discount	(9,703)
Carrying value	$ 42,297

Share Based Compensation, Share Option Valuation (Details) - Stock Options [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Stock Option Activity [Roll Forward]
Outstanding at beginning of period (in shares)	8,615,997
Granted (in shares)	1,023,366
Exercised (in shares)	(19,701)
Forfeited (in shares)	(226,841)
Outstanding at end of period (in shares)	9,392,821		8,615,997
Options vested and exercisable at end of period (in shares)	6,777,244
Options unvested at ending of period (in shares)	2,617,828
Weighted Average Exercise Price [Abstract]
Outstanding at beginning of period (in dollars per share)	$ 4.48
Granted (in dollars per share)	14.61
Exercised (in dollars per share)	1.92
forfeited (in dollars per share)	8.67
Outstanding at end of period (in dollars per share)	5.56		$ 4.48
Options vested and exercisable at end of period (in dollars per share)	2.17
Options unvested at ending (in dollars per share)	$ 14.35
Weighted-Average Remaining Contractual Term [Abstract]
Outstanding	7 years 6 months 18 days		7 years 6 months 4 days
Granted	9 years 9 months 14 days
Options vested and exercisable	6 years 11 months 1 day
Options unvested	9 years 2 months 12 days
Aggregate Intrinsic Value [Abstract]
Outstanding at beginning of period	$ 94,474
Exercised	801
Outstanding at end of period	114,173		$ 94,474
Options vested and exercisable	104,547
Options unvested	9,626
Intrinsic value of options exercised and exercisable	$ 114	$ 130
Weighted average grant date fair value of shares granted (in dollars per share)	$ 9.60	$ 12.84
Total fair value of options vested	$ 20,493	$ 24,724
Employees and Directors [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.61%	2.57%
Expected term	5 years 9 months 7 days	5 years 9 months 4 days
Expected volatility	74.60%	88.60%
Expected dividend yield	0.00%	0.00%
Exercise price (in dollars per share)	$ 14.61	$ 17.52
Fair value of common stock (in dollars per share)	$ 14.61	$ 17.52
Non-Employees [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.74%
Expected term	10 years
Expected volatility	83.79%
Expected dividend yield	0.00%
Exercise price (in dollars per share)	$ 18.75
Fair value of common stock (in dollars per share)	$ 18.75

Share Based Compensation, Share-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Stock-Based Compensation Expense [Abstract]
Stock-based compensation expense	$ 3,180	$ 5,382
Unrecognized share-based compensation cost	$ 24,900
Weighted average period expected to recognize unrecognized share-based compensation cost	2 years 25 days
Research and Development [Member]
Stock-Based Compensation Expense [Abstract]
Stock-based compensation expense	$ 1,565	2,207
General and Administrative [Member]
Stock-Based Compensation Expense [Abstract]
Stock-based compensation expense	$ 1,615	$ 3,175

Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	12 Months Ended
	Jan. 02, 2019	Dec. 28, 2018	Mar. 31, 2019	Dec. 31, 2018
Shareholders' Equity Disclosure [Abstract]
Shares repurchased (in shares)		100,000
Shares repurchased		$ 1,400	$ 727
Treasury stock			$ 0	$ (668)
Warrants outstanding (in shares)			14,102
Exercise price of warrants (in dollars per share)			$ 24.82
Warrants expiry date			Jun. 28, 2023
Warrants Exercisable for Common Shares [Member]
Shareholders' Equity Disclosure [Abstract]
Number of shares issuable on exercise of warrants outstanding (in shares)			14,102
Common Stock [Member]
Shareholders' Equity Disclosure [Abstract]
Shares repurchased (in shares)			0
Shares repurchased			$ 0
Number of shares settled (in shares)	50,000			50,000
Average price of shares (in dollars per share)	$ 14.50			$ 13.36
Prepaid Expense				$ 700
Treasury Stock [Member]
Shareholders' Equity Disclosure [Abstract]
Shares repurchased			$ 725

Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (19,451)	$ (15,343)
Denominator:
Weighted-average common shares outstanding - basic and diluted (in shares)	45,122,815	36,137,120
Net loss per share - basic and diluted	$ (0.43)	$ (0.42)
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	11,027,771	10,270,139
Shares Issuable upon Conversion of the 2021 Convertible Notes [Member]
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	1,620,848	1,620,948
Warrants Exercisable for Common Shares [Member]
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	14,102	14,102
Options to Purchase Common Shares [Member]
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Securities excluded from computation of diluted net loss per share (in shares)	9,392,821	8,635,089

Commitments and Contingencies (Details) $ in Thousands	3 Months Ended
Commitments and Contingencies (Details) $ in Thousands	Mar. 31, 2019 USD ($) ft² LeaseAgreement	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Operating Leases [Abstract]
Restricted cash	$ 1,525		$ 1,436
Lease cost [Abstract]
Operating lease cost	251
Total lease cost	251
Maturities of operating lease liabilities [Abstract]
2019 (remaining nine months)	818
2020	1,103
2021	894
2022	572
2023	74
Total lease payments	3,460
Less: interest	(399)
Total operating lease liabilities	3,061
Future annual minimum lease payment commitments [Abstract]
2019 (remaining nine months)	1,188
2020	1,970
2021	1,898
2022	1,757
2023	1,618
Thereafter	20,144
Total	28,575
Operating lease assets and liabilities [Abstract]
Operating right-of-use assets	2,647		0
Operating current lease liabilities	892		0
Operating noncurrent lease liabilities	2,169		0
Total operating lease liabilities	3,061
Cash paid for amounts included in the measurement of lease liabilities [Abstract]
Operating cash flows from operating leases	$ 270
Weighted-average remaining lease term - operating leases	3 years 3 months 18 days
Weighted-average discount rate - operating leases	7.77%
Rent expense	$ 200	$ 100
Property Subject to Operating Lease [Member]
Operating Leases [Abstract]
Restricted cash	$ 1,000		$ 1,400
NJ Lease Agreement [Member]
Operating Leases [Abstract]
Area of lease \| ft²	92,000
Area handed to commence term of lease \| ft²	72,000
Area handed over upon earlier of election to commence lease of additional space \| ft²	20,000
Term of lease agreement	15 years
Number of options to renew lease agreement \| LeaseAgreement	2
Term of renewal lease agreement	5 years
Estimated rent payments	$ 1,200
Percentage of annual increase in base rent	3.00%
Cash security deposit	$ 300
Maturities of operating lease liabilities [Abstract]
Total lease payments	$ 26,492
NJ Lease Agreement [Member] \| Maximum [Member]
Operating Leases [Abstract]
Period of time to commence lease of additional space	30 months

Strategic Research Collaboration (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2019	Mar. 31, 2019	Jan. 31, 2019
Collaboration Agreement [Abstract]
Funding in strategic research collaboration		$ 3,500
Percentage of funding due upon execution of the agreement		40.00%
Initial funding due upon the execution of the agreement		$ 1,400
Funding in Strategic Research Collaboration Upon Execution of Agreement Remaining Amount		$ 2,100
Accrued funding in strategic research collaboration			$ 1,400
Subsequent Events [Member]
Collaboration Agreement [Abstract]
Milestone payment made during period	$ (1,400)

Related Party Transactions (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Member of the Board of Directors - One [Member]
Related Party Transaction [Abstract]
Accrued consulting services	$ 4	$ 30
Business development consulting services expense (per quarter)	$ 28
Termination notice period for business development consulting services agreement	14 days
Member of the Board of Directors - Two [Member]
Related Party Transaction [Abstract]
Accrued consulting services	$ 28	$ 0

401(k) Savings Plan (Details) - USD ($) $ in Thousands	3 Months Ended
401(k) Savings Plan (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Mar. 31, 2018
401(k) Savings Plan [Abstract]
Percentage of matching employee contributions	4.00%
Matching employee contributions	$ 81	$ 30

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Apr. 18, 2019	Mar. 31, 2019	Mar. 31, 2018	Apr. 30, 2019
Shareholders' Equity Disclosure [Abstract]
Gross proceeds, offering amount		$ 0	$ 78,518
Common Stock [Member]
Shareholders' Equity Disclosure [Abstract]
Common stock shares issued (in shares)			6,325,000
Subsequent Event [Member] \| IPO [Member] \| Common Stock [Member]
Shareholders' Equity Disclosure [Abstract]
Common stock shares issued (in shares)	5,175,000
Number of shares issued from the exercise of the underwriters' option (in shares)	675,000
Public offering price (in dollars per share)	$ 17.50
Gross proceeds, offering amount	$ 90,600
Offering costs	4,600
Net proceeds, offering amount	$ 86,000
Grant award for clinical development support				$ 6,500