Consolidated Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended			73 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Operating expenses
Research and development	$ 32,608	$ 14,855	$ 10,963	$ 78,481
General and administrative	13,505	4,428	1,936	23,646
Total operating expenses	46,113	19,283	12,899	102,127
Loss from operations	(46,113)	(19,283)	(12,899)	(102,127)
Interest income	132	1		135
Interest expense		(128)	(393)	(863)
Other income (expense), net	(11,532)	913	75	(9,502)
Net loss and comprehensive loss	(57,513)	(18,497)	(13,217)	(112,357)
Deemed dividend related to beneficial conversion feature of convertible preferred stock	(25,559)			(25,559)
Accretion of Series A preferred stock to redemption value, net of extinguishment				1,098
Net loss attributable to common stockholders	$ (83,072)	$ (18,497)	$ (13,217)	$ (136,818)
Net loss per basic and diluted share attributable to common stockholders	$ (4.39)	$ (41.10)	$ (114.71)
Weighted-average common shares used to compute basic and diluted net loss per share	18,921,533	450,000	115,219