Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Operating expenses
General and administrative	$ (9,386)	$ (7,502)	$ (19,187)	$ (15,525)
Research and development	(13,304)	(10,627)	(25,613)	(22,245)
Loss from operations	(22,690)	(18,129)	(44,800)	(37,770)
Other income (expense), net	(482)	(505)	(1,030)	1,897
Net loss before income taxes	(23,172)	(18,634)	(45,830)	(35,873)
Income tax expense	(47)	(152)	(93)	(152)
Net loss	(23,219)	(18,786)	(45,923)	(36,025)
Net loss attributable to common stockholders—basic and diluted	$ (23,219)	$ (18,786)	$ (45,923)	$ (36,025)
Net loss per share attributable to common stockholders—basic and diluted (in USD per share)	$ (0.87)	$ (0.73)	$ (1.72)	$ (1.43)
Weighted average number of common shares outstanding—basic and diluted (shares)	26,773,337	25,841,530	26,748,301	25,225,521
Unrealized gain on available-for-sale investments	$ 58	$ 17	$ 169	$ 66
Comprehensive loss	$ (23,161)	$ (18,769)	$ (45,754)	$ (35,959)