Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2015	$ 182,760	$ 25	$ 558,251	$ 0	$ (375,516)
Beginning balance (shares) at Dec. 31, 2015		24,772
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(69,707)				(69,707)
Issuance of common stock under employee equity plans	350	$ 0	350
Issuance of common stock under employee equity plans (shares)		41
Stock-based compensation	7,353		7,353
Ending balance at Dec. 31, 2016	120,756	$ 25	565,954	0	(445,223)
Ending balance (shares) at Dec. 31, 2016		24,813
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(126,817)				(126,817)
Issuance of common stock, net of offering costs	290,591	$ 9	290,582
Issuance of common stock, net of offering costs (shares)		9,256
Issuance of common stock upon net exercise of common stock warrants		$ 0	0
Issuance of common stock upon net exercise of common stock warrants (shares)		26
Issuance of common stock under employee equity plans	10,836	$ 1	10,835
Issuance of common stock under employee equity plans (shares)		713
Stock-based compensation	6,155		6,155
Ending balance at Dec. 31, 2017	301,521	$ 35	873,526	0	(572,040)
Ending balance (shares) at Dec. 31, 2017		34,808
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(123,914)				(123,914)
Other Comprehensive Income	3			3
Issuance of common stock, net of offering costs	323,135	$ 7	323,128
Issuance of common stock, net of offering costs (shares)		6,740
Issuance of common stock under employee equity plans	7,994	$ 0	7,994
Issuance of common stock under employee equity plans (shares)		563
Shares repurchased for tax withholdings related to net share settlement of employee equity	(1,430)		(1,430)
Shares repurchased for tax withholdings related to net share settlement of employee equity (shares)		(33)
Stock-based compensation	15,492		15,492
Ending balance at Dec. 31, 2018	$ 522,801	$ 42	$ 1,218,710	$ 3	$ (695,954)
Ending balance (shares) at Dec. 31, 2018		42,078