Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Stockholders Equity, Beginning Balance at Dec. 31, 2015	$ 145,371	$ 30	$ 679,528	$ (534,187)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2015		30,058,105
Issuance of common stock for exercise of stock options	211		211
Issuance of common stock for exercise of stock options, Shares		312,841
Stock-based compensation	5,510		5,510
Vesting of early exercised stock options	23		23
Net loss	(47,867)			(47,867)
Stockholders Equity, Ending Balance at Dec. 31, 2016	103,248	$ 30	685,272	(582,054)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2016		30,370,946
Issuance of common stock for exercise of stock options	166		166
Issuance of common stock for exercise of stock options, Shares		176,641
Cumulative effect of adoption of new accounting standard			8	(8)
Stock-based compensation	5,905		5,905
Issuance of stock, value	27,422	$ 22	27,400
Issuance of stock, shares		21,847,636
Net loss	(42,015)			(42,015)
Stockholders Equity, Ending Balance at Dec. 31, 2017	94,726	$ 52	718,751	(624,077)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2017		52,395,223
Issuance of common stock for exercise of stock options	180		180
Issuance of common stock for exercise of stock options, Shares		120,742
Stock-based compensation	6,796		6,796
Issuance of stock, value	45,996	$ 22	45,974
Issuance of stock, shares		21,850,000
Issuance of common stock warrant	116		116
Net loss	(53,335)			(53,335)
Stockholders Equity, Ending Balance at Dec. 31, 2018	$ 94,479	$ 74	$ 771,817	$ (677,412)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2018		74,365,965