Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses
General and administrative	$ (30,635)	$ (20,103)	$ (10,287)
Research and development	(44,451)	(29,869)	(11,883)
Loss from operations	(75,086)	(49,972)	(22,170)
Other income (expense), net	862	1,839	(8,978)
Net loss before income taxes	(74,224)	(48,133)	(31,148)
Income tax expense	(139)	0	0
Net loss before income taxes	(74,363)	(48,133)	(31,148)
Net loss attributable to common stockholders—basic and diluted	$ (74,363)	$ (48,133)	$ (31,598)
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (2.88)	$ (2.00)	$ (6.38)
Weighted average number of common shares outstanding—basic and diluted	25,781,230	24,086,651	4,955,760
Unrealized loss on available-for-sale investments	$ (72)	$ (107)	$ 0
Comprehensive loss	$ (74,435)	$ (48,240)	$ (31,148)