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)		262,000
Issuance of common stock under employee purchase plan (shares)		14,000
Repurchase of common stock under Prepaid Forward contract (shares)		(1,859,000)
Ending Balance (shares) at Jun. 30, 2018		54,091,000
Beginning Balance, amount at Dec. 31, 2017		$ 56	$ 360,620	$ (241,972)	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options			2,757
Issuance of common stock under employee purchase plan			276
Repurchase of common stock under Prepaid Forward contract		(2)			(45,067)
Issuance of convertible note			53,282
Equity-based compensation			9,156
Net loss	$ (44,037)			(44,037)
Unrealized loss on available-for-sale securities	(55)					(55)
Foreign currency translation adjustment	(79)					(79)
Ending Balance, amount at Jun. 30, 2018	94,886	$ 54	426,091	(286,058)	(45,067)	(134)
Beginning Balance (shares) at Mar. 31, 2018		53,950,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options (shares)		134,000
Issuance of common stock under employee purchase plan (shares)		7,000
Ending Balance (shares) at Jun. 30, 2018		54,091,000
Beginning Balance, amount at Mar. 31, 2018		$ 54	414,262	(262,833)	(45,067)	59
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options			1,645
Issuance of common stock under employee purchase plan			142
Issuance of convertible note			6,668
Equity-based compensation			3,374
Net loss	(23,225)			(23,225)
Unrealized loss on available-for-sale securities	(2)					(2)
Foreign currency translation adjustment	(191)					(191)
Ending Balance, amount at Jun. 30, 2018	$ 94,886	$ 54	426,091	(286,058)	(45,067)	(134)
Beginning Balance (shares) at Dec. 31, 2018		54,232,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options (shares)	260,821	268,000
Issuance of common stock under employee purchase plan (shares)		12,000
Ending Balance (shares) at Jun. 30, 2019		54,512,000
Beginning Balance, amount at Dec. 31, 2018	$ 57,375	$ 54	432,885	(330,348)	(45,067)	(149)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options		1	4,369
Issuance of common stock under employee purchase plan			250
Equity-based compensation			6,353
Net loss	(42,536)			(42,536)
Unrealized loss on available-for-sale securities	219					219
Foreign currency translation adjustment	(26)					(26)
Ending Balance, amount at Jun. 30, 2019	26,005	$ 55	443,857	(372,884)	(45,067)	44
Beginning Balance (shares) at Mar. 31, 2019		54,454,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options (shares)		53,000
Issuance of common stock under employee purchase plan (shares)		5,000
Ending Balance (shares) at Jun. 30, 2019		54,512,000
Beginning Balance, amount at Mar. 31, 2019		$ 54	440,154	(352,069)	(45,067)	(104)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options		1	692
Issuance of common stock under employee purchase plan			111
Equity-based compensation			2,900
Net loss	(20,815)			(20,815)
Unrealized loss on available-for-sale securities	98					98
Foreign currency translation adjustment	50					50
Ending Balance, amount at Jun. 30, 2019	$ 26,005	$ 55	$ 443,857	$ (372,884)	$ (45,067)	$ 44