Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenues	$ 5,145	$ 11,686	$ 9,098
Operating expenses:
Research and development	63,764	73,768	61,932
General and administrative	19,211	20,916	12,932
Total operating expenses	82,975	94,684	74,864
Loss from operations	(77,830)	(82,998)	(65,766)
Other income (expense):
Interest income	350	42	17
Interest expense		(231)	(1,017)
Other income (expense)		(2)	24
Other income (expense), net	350	(191)	(976)
Net loss	$ (77,480)	$ (83,189)	$ (66,742)
Net loss per share-basic and diluted	$ (2.11)	$ (2.36)	$ (2.49)
Weighted-average number of common shares used in net loss per share-basic and diluted	36,704	35,261	26,807
Comprehensive loss	$ (77,480)	$ (83,189)	$ (66,742)