Nature of Business and Basis of Presentation - Schedule of Error Corrections (Details) - USD ($) $ in Thousands		3 Months Ended				9 Months Ended
	Jun. 24, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accumulated deficit		$ 497,007	$ 303,559	$ 164,209	$ 171,985	$ 497,007	$ 303,559	$ 489,734	$ 323,848	$ 320,443	$ 171,962
Deemed dividends on preferred stock and amortization of beneficial conversion feature				(2,352)	(2,856)
Redeemable preferred stock					35,471
Redemption of Series A Preferred Stock				(21,348)
Gain on redemption of preferred stock		0	0	9,075		0	9,075
Series A Redeemable Convertible Preferred Stock
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Deemed dividends on preferred stock and amortization of beneficial conversion feature				2,352	2,856
Redeemable preferred stock		0	0	0	35,471	0	0	0	0	0	32,615
Redemption of Series A Preferred Stock				(37,823)
Gain on redemption of preferred stock	$ 9,100			0
Series A Redeemable Convertible Preferred Stock | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Deemed dividends on preferred stock and amortization of beneficial conversion feature				5,550
Redeemable preferred stock					35,471
Redemption of Series A Preferred Stock				(50,096)
Gain on redemption of preferred stock				9,075
Series A Redeemable Convertible Preferred Stock | Immaterial Correction of an Error
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Deemed dividends on preferred stock and amortization of beneficial conversion feature				18,150
Gain on redemption of preferred stock				(18,150)
Series A Redeemable Convertible Preferred Stock | Reclassification
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Deemed dividends on preferred stock and amortization of beneficial conversion feature				(21,348)
Redemption of Series A Preferred Stock				12,273
Gain on redemption of preferred stock				9,075
Additional Paid-in Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accumulated deficit		516,936	322,592	185,556	193,840	516,936	322,592	511,589	346,676	340,291	194,687
Deemed dividends on preferred stock and amortization of beneficial conversion feature				(2,352)	(2,856)
Redemption of Series A Preferred Stock				(21,348)	(21,300)
Gain on redemption of preferred stock				9,075
Additional Paid-in Capital | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accumulated deficit			354,487	217,451	232,039		354,487	543,484	378,571	372,186	230,030
Deemed dividends on preferred stock and amortization of beneficial conversion feature				419
Accumulated Deficit
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accumulated deficit		$ (17,653)	(19,662)	(21,685)	(22,435)	$ (17,653)	(19,662)	(19,796)	(21,787)	(19,363)	(23,079)
Accumulated Deficit | Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accumulated deficit			$ (51,557)	(53,580)	(60,634)		$ (51,557)	$ (51,691)	$ (53,682)	$ (51,258)	$ (58,422)
Deemed dividends on preferred stock and amortization of beneficial conversion feature				(2,771)	$ (2,856)
Gain on redemption of preferred stock				$ 9,075