Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Stockholders' Equity Beginning Balance at Dec. 31, 2018	$ 522,801	$ 42	$ 1,218,710	$ 3	$ (695,954)
Stockholders' Equity - Beginning Balance (shares) at Dec. 31, 2018		42,078
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(35,202)				(35,202)
Other comprehensive income (loss)	370			370
Issuance of common stock under employee equity plans	5,293		5,293
Issuance of common stock under employee equity plans (shares)		380
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(606)		(606)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (shares)		(12)
Stock-based compensation	4,223		4,223
Stockholders' Equity Ending Balance at Mar. 31, 2019	496,879	$ 42	1,227,620	373	(731,156)
Stockholders' Equity - Ending Balance (shares) at Mar. 31, 2019		42,446
Stockholders' Equity Beginning Balance at Dec. 31, 2019	245,059	$ 45	1,360,092	379	(1,115,457)
Stockholders' Equity - Beginning Balance (shares) at Dec. 31, 2019		45,272
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(25,800)				(25,800)
Other comprehensive income (loss)	(167)			(167)
Issuance of common stock, net of offering costs	221,708	$ 10	221,698
Issuance of common stock, net of offering costs (shares)		9,798
Issuance of common stock under employee equity plans	3,882		3,882
Issuance of common stock under employee equity plans (shares)		297
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(569)		(569)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (shares)		(26)
Stock-based compensation	6,394		6,394
Stockholders' Equity Ending Balance at Mar. 31, 2020	$ 450,507	$ 55	$ 1,591,497	$ 212	$ (1,141,257)
Stockholders' Equity - Ending Balance (shares) at Mar. 31, 2020		55,341