Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]
Beginning balance (in shares) at Dec. 31, 2017		44,054	487
Beginning balance at Dec. 31, 2017	$ 24,153	$ 441	$ (4,764)	$ 463,428	$ (39)	$ (434,913)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	13,575			13,575
Issuance of common stock related to follow-on public offering (in shares)		1,500
Issuance of common stock related to follow-on public offering	30,907	$ 15		30,892
Issuance of common stock under Employee Stock Purchase Plan (in shares)		124
Issuance of common stock under Employee Stock Purchase Plan	1,632	$ 1		1,631
Vesting of restricted stock units (in shares)		425
Vesting of restricted stock units	0	$ 4		(4)
Forfeiture of restricted stock awards (in shares)		(3)
Forfeiture of restricted stock awards	0
Shares withheld for employee taxes (in shares)		(42)
Shares withheld for employee taxes	(1,005)	$ (1)		(1,004)
Issuance of common stock upon exercise of stock options (in shares)		681
Issuance of common stock upon exercise of stock options	5,087	$ 7		5,080
Net unrealized gain on investments	15				15
Net loss	(30,694)					(30,694)
Ending balance (in shares) at Jun. 30, 2018		46,739	487
Ending balance at Jun. 30, 2018	69,543	$ 467	$ (4,764)	513,598	(24)	(439,734)
Beginning balance (in shares) at Mar. 31, 2018		46,199	487
Beginning balance at Mar. 31, 2018	73,922	$ 462	$ (4,764)	503,669	(44)	(425,401)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	7,350			7,350
Vesting of restricted stock units (in shares)		269
Vesting of restricted stock units	0	$ 2		(2)
Shares withheld for employee taxes (in shares)		(22)
Shares withheld for employee taxes	(543)			(543)
Issuance of common stock upon exercise of stock options (in shares)		293
Issuance of common stock upon exercise of stock options	3,127	$ 3		3,124
Net unrealized gain on investments	20				20
Net loss	(14,333)					(14,333)
Ending balance (in shares) at Jun. 30, 2018		46,739	487
Ending balance at Jun. 30, 2018	69,543	$ 467	$ (4,764)	513,598	(24)	(439,734)
Beginning balance (in shares) at Dec. 31, 2018		47,600	487
Beginning balance at Dec. 31, 2018	87,319	$ 476	$ (4,764)	556,223	(31)	(464,585)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	19,064			19,064
Issuance of common stock under Employee Stock Purchase Plan (in shares)		111
Issuance of common stock under Employee Stock Purchase Plan	2,634	$ 1		2,633
Vesting of restricted stock units (in shares)		595
Vesting of restricted stock units	0	$ 6		(6)
Shares withheld for employee taxes (in shares)		(58)
Shares withheld for employee taxes	(2,839)	$ (1)		(2,838)
Issuance of common stock upon exercise of stock options (in shares)		550
Issuance of common stock upon exercise of stock options	6,057	$ 6		6,051
Net unrealized gain on investments	382				382
Net loss	(25,093)					(25,093)
Ending balance (in shares) at Jun. 30, 2019		48,798	487
Ending balance at Jun. 30, 2019	87,524	$ 488	$ (4,764)	581,127	351	(489,678)
Beginning balance (in shares) at Mar. 31, 2019		48,158	487
Beginning balance at Mar. 31, 2019	88,851	$ 482	$ (4,764)	569,229	162	(476,258)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	10,430			10,430
Vesting of restricted stock units (in shares)		351
Vesting of restricted stock units	0	$ 3		(3)
Shares withheld for employee taxes (in shares)		(36)
Shares withheld for employee taxes	(1,859)	$ (1)		(1,858)
Issuance of common stock upon exercise of stock options (in shares)		325
Issuance of common stock upon exercise of stock options	3,333	$ 4		3,329
Net unrealized gain on investments	189				189
Net loss	(13,420)					(13,420)
Ending balance (in shares) at Jun. 30, 2019		48,798	487
Ending balance at Jun. 30, 2019	$ 87,524	$ 488	$ (4,764)	$ 581,127	$ 351	$ (489,678)