CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit	Total
Balance at beginning of period at Dec. 31, 2017		$ 384,943	$ (246)	$ (159,435)	$ 225,262
Balance at beginning of period (in shares) at Dec. 31, 2017	30,856,505
Increase (Decrease) in Stockholders' Equity
Exercise of stock options and vesting of RSUs		378			378
Exercise of stock options and vesting of RSUs, shares	49,124
Unrealized gain (loss) on marketable securities			(65)		(65)
Foreign currency translation adjustment			(17)		(17)
Stock-based compensation expense		5,143			5,143
Net loss				(30,229)	(30,229)
Balance at end of period at Mar. 31, 2018		390,464	(328)	(189,664)	200,472
Balance at end of period (in shares) at Mar. 31, 2018	30,905,629
Balance at beginning of period at Dec. 31, 2017		384,943	(246)	(159,435)	225,262
Balance at beginning of period (in shares) at Dec. 31, 2017	30,856,505
Increase (Decrease) in Stockholders' Equity
Unrealized gain (loss) on marketable securities					111
Net loss					(94,187)
Balance at end of period at Sep. 30, 2018		400,066	(116)	(253,622)	146,328
Balance at end of period (in shares) at Sep. 30, 2018	30,991,060
Balance at beginning of period at Mar. 31, 2018		390,464	(328)	(189,664)	200,472
Balance at beginning of period (in shares) at Mar. 31, 2018	30,905,629
Increase (Decrease) in Stockholders' Equity
Exercise of stock options and vesting of RSUs		(440)			(440)
Exercise of stock options and vesting of RSUs, shares	59,667
Unrealized gain (loss) on marketable securities			111		111
Foreign currency translation adjustment			29		29
Stock-based compensation expense		5,249			5,249
Net loss				(31,218)	(31,218)
Balance at end of period at Jun. 30, 2018		395,273	(188)	(220,882)	174,203
Balance at end of period (in shares) at Jun. 30, 2018	30,965,296
Increase (Decrease) in Stockholders' Equity
Exercise of stock options and vesting of RSUs		86			86
Exercise of stock options and vesting of RSUs, shares	25,764
Unrealized gain (loss) on marketable securities			65		65
Foreign currency translation adjustment			7		7
Stock-based compensation expense		4,707			4,707
Net loss				(32,740)	(32,740)
Balance at end of period at Sep. 30, 2018		400,066	(116)	(253,622)	146,328
Balance at end of period (in shares) at Sep. 30, 2018	30,991,060
Balance at beginning of period at Dec. 31, 2018		507,366	(69)	(292,173)	$ 215,124
Balance at beginning of period (in shares) at Dec. 31, 2018	41,210,725				41,210,725
Increase (Decrease) in Stockholders' Equity
Exercise of stock options and vesting of RSUs		(188)			$ (188)
Exercise of stock options and vesting of RSUs, shares	58,918
Unrealized gain (loss) on marketable securities			34		34
Foreign currency translation adjustment			(14)		(14)
Stock-based compensation expense		4,862			4,862
Net loss				(37,565)	(37,565)
Balance at end of period at Mar. 31, 2019		512,040	(49)	(329,738)	182,253
Balance at end of period (in shares) at Mar. 31, 2019	41,269,643
Balance at beginning of period at Dec. 31, 2018		507,366	(69)	(292,173)	$ 215,124
Balance at beginning of period (in shares) at Dec. 31, 2018	41,210,725				41,210,725
Increase (Decrease) in Stockholders' Equity
Unrealized gain (loss) on marketable securities					$ 41
Net loss					(142,760)
Balance at end of period at Sep. 30, 2019		520,209	(1)	(434,933)	$ 85,275
Balance at end of period (in shares) at Sep. 30, 2019	41,380,811				41,380,811
Balance at beginning of period at Mar. 31, 2019		512,040	(49)	(329,738)	$ 182,253
Balance at beginning of period (in shares) at Mar. 31, 2019	41,269,643
Increase (Decrease) in Stockholders' Equity
Exercise of stock options and vesting of RSUs		(18)			(18)
Exercise of stock options and vesting of RSUs, shares	8,927
Unrealized gain (loss) on marketable securities			30		30
Foreign currency translation adjustment			27		27
Stock-based compensation expense		4,814			4,814
Net loss				(49,876)	(49,876)
Balance at end of period at Jun. 30, 2019		516,836	8	(379,614)	137,230
Balance at end of period (in shares) at Jun. 30, 2019	41,278,570
Increase (Decrease) in Stockholders' Equity
Exercise of stock options and vesting of RSUs		53			53
Exercise of stock options and vesting of RSUs, shares	102,241
Unrealized gain (loss) on marketable securities			(23)		(23)
Foreign currency translation adjustment			14		14
Stock-based compensation expense		3,320			3,320
Net loss				(55,319)	(55,319)
Balance at end of period at Sep. 30, 2019		$ 520,209	$ (1)	$ (434,933)	$ 85,275
Balance at end of period (in shares) at Sep. 30, 2019	41,380,811				41,380,811