Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($)	Jan. 01, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Recently Adopted Accounting Pronouncements
Additional paid-in capital		$ 575,365,000				$ 543,040,000				$ 17,277,000
Accumulated deficit		(268,268,000)			$ (149,282,000)	(150,082,000)				(60,979,000)
Change in fair value of the unvested awards	$ 100,000
Financial impact on the Company’s condensed consolidated balance sheet upon the adoption of ASUs and the cumulative effect adjustment
Operating lease, right-of-use-asset		47,911,000			1,751,000
Property, plant and equipment, net		52,478,000			17,654,000	62,796,000
Deferred rent						143,000
Accrued expenses and other current liabilities					7,667,000	12,118,000
Lease liability, net of current portion						41,441,000
Operating lease liabilities, current portion		6,998,000			616,000	4,502,000
Operating lease liabilities, net of current portion		41,439,000			1,226,000	41,441,000
Stockholders Equity		$ 307,343,000	$ 343,644,000	$ 371,636,000		$ 393,008,000	$ 411,007,000	$ (70,709,000)	$ (55,526,000)	$ (43,687,000)
Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Weighted average grant-date fair value (per share)	$ 5.88
Risk-free interest rate	2.30%
Expected volatility	74.00%
Expected term (in years)	6 years 1 month 6 days
Common stock value	$ 6.28
Restricted Common Stock | Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Number of outstanding options	4,767,014
Stock options | Non-employees
Assumptions used to determine the fair value of outstanding awards granted
Number of outstanding options	330,917
ASU 2018-07
Recently Adopted Accounting Pronouncements
Additional paid-in capital	$ 17,185,000
Accumulated deficit	(60,887,000)
ASU 2016-02
Recently Adopted Accounting Pronouncements
Change in fair value of the unvested awards				$ 800,000
ASU 2016-02 | Adjustments
Recently Adopted Accounting Pronouncements
Accumulated deficit					800,000
Financial impact on the Company’s condensed consolidated balance sheet upon the adoption of ASUs and the cumulative effect adjustment
Operating lease, right-of-use-asset					1,751,000
Property, plant and equipment, net					(45,142,000)
Deferred rent					(143,000)
Accrued expenses and other current liabilities					(4,451,000)
Lease liability, net of current portion					(41,441,000)
Operating lease liabilities, current portion					616,000
Operating lease liabilities, net of current portion					$ 1,226,000
Adjustments for New Accounting Principle, Early Adoption | ASU 2018-07
Recently Adopted Accounting Pronouncements
Additional paid-in capital	(92,000)
Accumulated deficit	$ 92,000