Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Receivables from Exercises of Options	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2018	$ 34,306	$ 4	$ 1,236,355		$ (12)	$ (1,202,041)
Beginning Balance (in shares) at Dec. 31, 2018		38,325,037
Stock-based compensation	33,578		33,578
Shares issued or restricted stock units vested under employee stock plans	1,276		1,276
Shares issued or restricted stock units vested under employee stock plans (in shares)		413,670
Unrealized gain (loss) on available-for- sale securities	122				122
Net income (loss)	(47,511)					(47,511)
Ending Balance at Jun. 30, 2019	21,771	$ 4	1,271,209		110	(1,249,552)
Ending Balance (in shares) at Jun. 30, 2019		38,738,707
Beginning Balance at Mar. 31, 2019	43,471	$ 4	1,255,586	$ (11)	20	(1,212,128)
Beginning Balance (in shares) at Mar. 31, 2019		38,573,118
Stock-based compensation	15,440		15,440
Shares issued or restricted stock units vested under employee stock plans	194		183	$ 11
Shares issued or restricted stock units vested under employee stock plans (in shares)		165,589
Unrealized gain (loss) on available-for- sale securities	90				90
Net income (loss)	(37,424)					(37,424)
Ending Balance at Jun. 30, 2019	21,771	$ 4	1,271,209		110	(1,249,552)
Ending Balance (in shares) at Jun. 30, 2019		38,738,707
Beginning Balance at Dec. 31, 2019	$ 17,463	$ 4	1,295,033		62	(1,277,636)
Beginning Balance (in shares) at Dec. 31, 2019	39,203,304	39,203,304
Stock-based compensation	$ 19,537		19,537
Shares issued or restricted stock units vested under employee stock plans	2		2
Shares issued or restricted stock units vested under employee stock plans (in shares)		423,613
Reclassification of gain on available-for-sale securities	3				3
Unrealized gain (loss) on available-for- sale securities	(64)				(64)
Net income (loss)	(13,538)					(13,538)
Ending Balance at Jun. 30, 2020	$ 23,403	$ 4	1,314,572		1	(1,291,174)
Ending Balance (in shares) at Jun. 30, 2020	39,626,917	39,626,917
Beginning Balance at Mar. 31, 2020	$ 9,377	$ 4	1,303,940		2	(1,294,569)
Beginning Balance (in shares) at Mar. 31, 2020		39,317,221
Stock-based compensation	10,630		10,630
Shares issued or restricted stock units vested under employee stock plans	2		2
Shares issued or restricted stock units vested under employee stock plans (in shares)		309,696
Unrealized gain (loss) on available-for- sale securities	(1)				(1)
Net income (loss)	3,395					3,395
Ending Balance at Jun. 30, 2020	$ 23,403	$ 4	$ 1,314,572		$ 1	$ (1,291,174)
Ending Balance (in shares) at Jun. 30, 2020	39,626,917	39,626,917