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]	Accumulated Other Comprehensive Gain (Loss) [Member]	Accumulated Deficit [Member]
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	8,634			8,634
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)		244
Vesting of restricted stock units	0	$ 3		(3)
Shares withheld for employee taxes (in shares)		(22)
Shares withheld for employee taxes	(980)			(980)
Issuance of common stock upon exercise of stock options (in shares)		225
Issuance of common stock upon exercise of stock options	2,724	$ 2		2,722
Net unrealized gain (loss) on investments	193				193
Net loss	(11,673)					(11,673)
Ending balance (in shares) at Mar. 31, 2019		48,158	487
Ending balance at Mar. 31, 2019	88,851	$ 482	$ (4,764)	569,229	162	(476,258)
Beginning balance (in shares) at Dec. 31, 2019		49,911	487
Beginning balance at Dec. 31, 2019	83,168	$ 499	$ (4,764)	605,650	213	(518,430)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	12,965			12,965
Issuance of common stock under employee stock purchase plan (in shares)		102
Issuance of common stock under employee stock purchase plan	3,346	$ 1		3,345
Vesting of restricted stock units (in shares)		308
Vesting of restricted stock units	0	$ 3		(3)
Shares withheld for employee taxes (in shares)		(27)
Shares withheld for employee taxes	(1,533)	$ 0		(1,533)
Issuance of common stock upon exercise of stock options (in shares)		125
Issuance of common stock upon exercise of stock options	1,569	$ 1		1,568
Net unrealized gain (loss) on investments	(291)				(291)
Net loss	(22,924)					(22,924)
Ending balance (in shares) at Mar. 31, 2020		50,419	487
Ending balance at Mar. 31, 2020	$ 76,300	$ 504	$ (4,764)	$ 621,992	$ (78)	$ (541,354)