CONDENSED CONSOLIDATEDSTATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Contributed Capital	Accumulated Deficit	Treasury stock	Accumulated Other Comprehensive Income (Loss)
Beginning Balance, amount at Dec. 31, 2017	$ 118,704	$ 56	$ 360,620	$ (241,972)	$ 0	$ 0
Beginning Balance (shares) at Dec. 31, 2017		55,674,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(20,812)			(20,812)
Exercise of options	1,112		1,112
Exercise of options (shares)		128,000
Issuance of common stock under employee purchase plan	134		134
Issuance of common stock under employee purchase plan (shares)		7,000
Unrealized loss on available-for-sale securities	(53)					(53)
Foreign currency translation adjustment	112					112
Repurchase of common stock under Prepaid Forward contract	(45,069)	$ (2)			(45,067)
Repurchase of common stock under Prepaid Forward contract (shares)		(1,859,000)
Issuance of convertible note	46,614		46,614
Equity-based compensation	5,782		5,782
Ending Balance, amount at Mar. 31, 2018	106,475	$ 54	414,262	(262,833)	(45,067)	59
Ending Balance (shares) at Mar. 31, 2018		53,950,000
Beginning Balance, amount at Dec. 31, 2018	57,375	$ 54	432,885	(330,348)	(45,067)	(149)
Beginning Balance (shares) at Dec. 31, 2018		54,232,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(21,721)			(21,721)
Exercise of options	$ 3,677		3,677
Exercise of options (shares)	208,044	215,000
Issuance of common stock under employee purchase plan	$ 139		139
Issuance of common stock under employee purchase plan (shares)		7,000
Unrealized loss on available-for-sale securities	121					121
Foreign currency translation adjustment	(76)					(76)
Equity-based compensation	3,453		3,453
Ending Balance, amount at Mar. 31, 2019	$ 42,968	$ 54	$ 440,154	$ (352,069)	$ (45,067)	$ (104)
Ending Balance (shares) at Mar. 31, 2019		54,454,000