Condensed Consolidated Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		36 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014
Operating expenses:
Research and development	$ 4,652	$ 2,680	$ 27,947
General and administrative	2,998	2,495	24,990
Total operating expenses	7,650	5,175	52,937
Loss from operations	(7,650)	(5,175)	(52,937)
Interest (expense) income, net	(57)	18	52
Other income (expense), net	(10)		(10)
Loss from equity method investment	(97)		(97)
Net loss	(7,814)	(5,157)	(52,992)
Accretion of convertible preferred stock to redemption value			(101)
Net loss applicable to common stockholders	(7,814)	(5,157)	(53,093)
Net loss per share applicable to common stockholders-basic and diluted (in dollars per share)	$ (0.41)	$ (0.39)	$ (5.73)
Weighted average number of common shares used in net loss per share applicable to common stockholders-basic and diluted (in shares)	19,214	13,345	9,270
Net loss	(7,814)	(5,157)	(52,992)
Other comprehensive loss:
Unrealized gains (losses) on available-for-sale securities	(13)	7	(3)
Comprehensive loss	(7,827)	(5,150)	(52,995)
Non-cash stock-based compensation expenses included in operating expenses are as follows:
Stock-based compensation expense	867	837	7,689
Research and development
Non-cash stock-based compensation expenses included in operating expenses are as follows:
Stock-based compensation expense	910	426	4,683
General and administrative
Non-cash stock-based compensation expenses included in operating expenses are as follows:
Stock-based compensation expense	$ (43)	$ 411	$ 3,006