Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating expenses
General and administrative	$ (44,478)	$ (30,635)	$ (20,103)
Research and development	(52,394)	(44,451)	(29,869)
Loss from operations	(96,872)	(75,086)	(49,972)
Other income (expense), net	(1,994)	862	1,839
Net loss before income taxes	(98,866)	(74,224)	(48,133)
Income tax expense	(193)	(139)	0
Net loss	(99,059)	(74,363)	(48,133)
Net loss attributable to common stockholders—basic and diluted	$ (99,059)	$ (74,363)	$ (48,133)
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (3.40)	$ (2.88)	$ (2.00)
Weighted average number of common shares outstanding—basic and diluted (in shares)	29,135,583	25,781,230	24,086,651
Net loss	$ (99,059)	$ (74,363)	$ (48,133)
Unrealized gain/(loss) on available-for-sale investments	111	(72)	(107)
Comprehensive loss	$ (98,948)	$ (74,435)	$ (48,240)