Summarized Quarterly Data (Unaudited) - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended							6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Net loss	$ (33,892)	$ (36,997)	$ (26,233)	$ (12,105)	$ (10,872)	$ (11,916)	$ (11,451)	$ (63,230)	$ (97,122)	$ (138,696)	$ (46,344)	$ (24,197)
Stock-based compensation			5,794					6,377	6,801	7,748	2,755	2,722
Accrued expenses and other current liabilities			10,399					13,089	26,525	26,512	938	4,839
Net cash used in operating activities			1,866					(27,624)	(42,335)	(66,698)	(34,304)	(13,189)
Net cash used in investing activities			(1,201)					(1,257)	(20,252)	(76,089)	(416,722)	(225,279)
Net cash provided by financing activities			10,923					15,967	92,602	139,482	465,607	253,717
Net (decrease)/ increase in cash, cash equivalents and restricted cash			11,588					(12,914)	30,015	(3,305)	14,581	15,249
Cash, cash equivalents and restricted cash, beginning of year	40,141	64,643	53,055				$ 38,474	53,055	53,055	53,055	38,474	23,225
Cash, cash equivalents and restricted cash, end of year	83,070	40,141	64,643	53,055				40,141	83,070	49,750	53,055	$ 38,474
Scenario, Previously Reported
Condensed Financial Statements, Captions [Line Items]
Net loss	(33,716)	(37,298)	(20,911)					(58,209)	(91,925)
Stock-based compensation			697					1,449	2,042
Accrued expenses and other current liabilities			10,174					12,996	26,087
Net cash used in operating activities			1,866					(27,624)	(42,335)
Net cash used in investing activities			(1,201)					(1,257)	(20,252)
Net cash provided by financing activities			10,923					15,967	92,602
Net (decrease)/ increase in cash, cash equivalents and restricted cash			11,588					(12,914)	30,015
Cash, cash equivalents and restricted cash, beginning of year	40,141	64,643	53,055					53,055	53,055	53,055
Cash, cash equivalents and restricted cash, end of year	83,070	40,141	64,643	53,055				40,141	83,070		53,055
Restatement Adjustment
Condensed Financial Statements, Captions [Line Items]
Net loss	(176)	301	(5,322)					(5,021)	(5,197)
Stock-based compensation			5,097					4,928	4,759
Accrued expenses and other current liabilities			225					93	438
Net cash used in operating activities			0					0	0
Net cash used in investing activities			0					0	0
Net cash provided by financing activities			0					0	0
Net (decrease)/ increase in cash, cash equivalents and restricted cash			0					0	0
Cash, cash equivalents and restricted cash, beginning of year	0	0	0					0	0	$ 0
Cash, cash equivalents and restricted cash, end of year	$ 0	$ 0	$ 0	$ 0				$ 0	$ 0		$ 0