Consolidated Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended		33 Months Ended
	Dec. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013
Operating expenses:
Research and development	$ 1,170	$ 15,802	$ 6,323	$ 23,295
General and administrative	1,454	13,332	7,206	21,992
Total operating expenses	2,624	29,134	13,529	45,287
Loss from operations	(2,624)	(29,134)	(13,529)	(45,287)
Interest income		90	19	109
Net loss	(2,624)	(29,044)	(13,510)	(45,178)
Accretion of convertible preferred stock to redemption value	(101)			(101)
Net loss applicable to common stockholders	(2,725)	(29,044)	(13,510)	(45,279)
Net loss per share applicable to common stockholders-basic and diluted (in dollars per share)	$ (3.00)	$ (1.80)	$ (2.33)	$ (5.42)
Weighted average number of common shares used in net loss per share applicable to common stockholders-basic and diluted (in shares)	909	16,160	5,810	8,350
Net loss	(2,624)	(29,044)	(13,510)	(45,178)
Other comprehensive loss:
Unrealized gains / (losses) on available-for-sale securities		16	(6)	10
Comprehensive loss	(2,624)	(29,028)	(13,516)	(45,168)
Non-cash stock-based compensation expenses included in operating expenses are as follows:
Stock-based compensation expense	346	5,094	1,382	6,822
Research and development
Non-cash stock-based compensation expenses included in operating expenses are as follows:
Stock-based compensation expense	269	2,361	1,143	3,773
General and administrative
Non-cash stock-based compensation expenses included in operating expenses are as follows:
Stock-based compensation expense	$ 77	$ 2,733	$ 239	$ 3,049