CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Contributed capital	Accumulated deficit	Treasury stock	Accumulated other comprehensive (loss) income
Beginning Balance (shares) at Dec. 31, 2017		55,674,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options (shares)		360,000
Issuance of common stock under employee purchase plan (shares)		22,000
Repurchase of common stock under Prepaid Forward contract (shares)		(1,859,000)
Ending Balance (shares) at Sep. 30, 2018		54,197,000
Beginning Balance, amount at Dec. 31, 2017		$ 56	$ 360,620	$ (241,972)	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from issuance of common stock	$ 447
Exercise of options			3,710
Repurchase of common stock under Prepaid Forward contract		(2)			(45,067)
Issuance of common stock under employee purchase plan			447
Issuance of convertible note			53,283
Equity-based compensation			12,674
Net loss	(66,135)			(66,135)
Unrealized loss on available-for-sale securities	(54)					(54)
Foreign currency translation adjustment	(105)					(105)
Ending Balance, amount at Sep. 30, 2018	77,406	$ 54	430,734	(308,156)	(45,067)	(159)
Beginning Balance (shares) at Jun. 30, 2018		54,091,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options (shares)		98,000
Issuance of common stock under employee purchase plan (shares)		8,000
Ending Balance (shares) at Sep. 30, 2018		54,197,000
Beginning Balance, amount at Jun. 30, 2018		$ 54	426,091	(286,058)	(45,067)	(134)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options			954
Issuance of common stock under employee purchase plan			171
Equity-based compensation			3,518
Net loss	(22,098)			(22,098)
Unrealized loss on available-for-sale securities	1					1
Foreign currency translation adjustment	(26)					(26)
Ending Balance, amount at Sep. 30, 2018	$ 77,406	$ 54	430,734	(308,156)	(45,067)	(159)
Beginning Balance (shares) at Dec. 31, 2018		54,232,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from issuance of common stock (shares)		56,000
Exercise of options (shares)	327,215	334,000
Issuance of common stock under employee purchase plan (shares)		19,000
Ending Balance (shares) at Sep. 30, 2019		54,641,000
Beginning Balance, amount at Dec. 31, 2018	$ 57,375	$ 54	432,885	(330,348)	(45,067)	(149)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from issuance of common stock	1,363
Exercise of options		1	4,879
Issuance of common stock under employee purchase plan			362
Equity-based compensation			9,483
Net loss	(62,970)			(62,970)
Unrealized loss on available-for-sale securities	229					229
Foreign currency translation adjustment	(139)					(139)
Ending Balance, amount at Sep. 30, 2019	10,220	$ 55	448,609	(393,318)	(45,067)	(59)
Beginning Balance (shares) at Jun. 30, 2019		54,512,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Proceeds from issuance of common stock (shares)		56,000
Proceeds from issuance of common stock			1,000
Exercise of options (shares)		66,000
Issuance of common stock under employee purchase plan (shares)		7,000
Ending Balance (shares) at Sep. 30, 2019		54,641,000
Beginning Balance, amount at Jun. 30, 2019		$ 55	443,857	(372,884)	(45,067)	44
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options			510
Issuance of common stock under employee purchase plan			112
Equity-based compensation			3,130
Net loss	(20,434)			(20,434)
Unrealized loss on available-for-sale securities	10					10
Foreign currency translation adjustment	(113)					(113)
Ending Balance, amount at Sep. 30, 2019	$ 10,220	$ 55	$ 448,609	$ (393,318)	$ (45,067)	$ (59)