Condensed Statements of Operations and Comprehensive Loss (Unaudited) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended		67 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014
Operating expenses
Research and development	$ 5,622	$ 3,326	$ 10,491	$ 6,825	$ 56,364
General and administrative	2,877	656	5,591	1,338	15,732
Total operating expenses	8,499	3,982	16,082	8,163	72,096
Loss from operations	(8,499)	(3,982)	(16,082)	(8,163)	(72,096)
Interest income	40		40		43
Interest expense				(128)	(863)
Other income (expense), net	(141)	327	(11,984)	617	(9,954)
Net loss and comprehensive loss	(8,600)	(3,655)	(28,026)	(7,674)	(82,870)
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	$ (8,600)	$ (3,655)	$ (53,585)	$ (7,674)	$ (107,331)
Net loss per basic and diluted share attributable to common stockholders	$ (0.36)	$ (24.74)	$ (3.95)	$ (54.59)
Weighted-average common shares used to compute basic and diluted net loss per share	24,194,808	147,713	13,550,582	140,565