CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Beginning Balance (in shares) at Dec. 31, 2015		41,540	402
Beginning Balance at Dec. 31, 2015	$ 68,075	$ 415	$ (3,526)	$ 411,524	$ 0	$ (340,338)
Stock-based compensation expense	17,316			17,316
Issuance of common stock under ESPP (in shares)		346
Issuance of common stock under ESPP	3,724	$ 4		3,720
Vesting of restricted stock units (in shares)		91
Vesting of restricted stock units	0	$ 1		(1)
Forfeiture of restricted stock awards (in shares)		(27)
Forfeiture of restricted stock awards	0
Shares withheld for employee taxes (in shares)		(74)	(62)
Shares withheld for employee taxes	(1,026)	$ (1)	$ (865)	(160)
Excess tax benefit from stock-based compensation	87			87
Issuance of common stock upon exercise of stock options (in shares)		679
Issuance of common stock upon exercise of stock options	2,881	$ 7		2,874
Net unrealized gain on investments	(19)				(19)
Net loss	(49,000)					(49,000)
Ending Balance (in shares) at Dec. 31, 2016		42,555	464
Ending Balance at Dec. 31, 2016	42,038	$ 426	$ (4,391)	435,360	(19)	(389,338)
Stock-based compensation expense	19,541			19,541
Issuance of common stock under ESPP (in shares)		247
Issuance of common stock under ESPP	2,914	$ 3		2,911
Vesting of restricted stock units (in shares)		436
Vesting of restricted stock units	0	$ 4		(4)
Forfeiture of restricted stock awards (in shares)		(21)
Forfeiture of restricted stock awards	0
Shares withheld for employee taxes (in shares)		(50)	(23)
Shares withheld for employee taxes	(698)	$ (1)	$ (373)	(324)
Issuance of common stock upon exercise of stock options (in shares)		887
Issuance of common stock upon exercise of stock options	5,848	$ 9		5,839
Net unrealized gain on investments	(20)				(20)
Net loss	(45,470)					(45,470)
Ending Balance (in shares) at Dec. 31, 2017		44,054	487
Ending Balance at Dec. 31, 2017	24,153	$ 441	$ (4,764)	463,428	(39)	(434,913)
Stock-based compensation expense	27,593			27,593
Equity component of convertible senior notes, net	52,194			52,194
Purchase of capped calls related to convertible senior notes	(26,910)			(26,910)
Issuance of common stock related to secondary offering (in shares)		1,500
Issuance of common stock related to secondary offering	30,907	$ 15		30,892
Issuance of common stock under ESPP (in shares)		219
Issuance of common stock under ESPP	3,637	$ 2		3,635
Vesting of restricted stock units (in shares)		973
Vesting of restricted stock units	0	$ 10		(10)
Forfeiture of restricted stock awards (in shares)		(3)
Forfeiture of restricted stock awards	0
Shares withheld for employee taxes (in shares)		(88)
Shares withheld for employee taxes	(2,197)	$ (1)		(2,196)
Issuance of common stock upon exercise of stock options (in shares)		945
Issuance of common stock upon exercise of stock options	7,606	$ 9		7,597
Net unrealized gain on investments	8				8
Net loss	(55,545)					(55,545)
Ending Balance (in shares) at Dec. 31, 2018		47,600	487
Ending Balance at Dec. 31, 2018	$ 87,319	$ 476	$ (4,764)	$ 556,223	$ (31)	$ (464,585)