Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development	$ 43,016	$ 23,263	$ 8,902
General and administrative	18,497	14,677	7,031
Total operating expenses	61,513	37,940	15,933
Operating loss	(61,513)	(37,940)	(15,933)
Other income (expense):
Interest income (expense), net	510	206	(704)
Extinguishment of debt and other liabilities			2,420
Other income	287	700	1,050
Net loss	(60,716)	(37,034)	(13,167)
Reconciliation of net loss to net loss applicable to common stockholders:
Net loss	(60,716)	(37,034)	(13,167)
Accretion on preferred stock		(86,899)	(55,886)
Net loss applicable to common stockholders	$ (60,716)	$ (123,933)	$ (69,053)
Net loss per share applicable to common stockholders—basic and diluted	$ (2.29)	$ (8.04)	$ (126.94)
Weighted-average number of common shares used in net loss per share applicable to common stockholders—basic and diluted	26,469,170	15,406,386	544,002
Comprehensive loss:
Net loss	$ (60,716)	$ (37,034)	$ (13,167)
Other comprehensive loss - unrealized loss on securities	(234)	(56)
Comprehensive loss	$ (60,950)	$ (37,090)	$ (13,167)