Consolidated Statements of Comprehensive Loss (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement of Comprehensive Income [Abstract]
Total revenue
Operating expenses:
Research and development	(17,709)	(9,597)	(49,330)	(15,594)
General and administrative	(3,249)	(1,614)	(8,635)	(4,240)
Total operating expenses	(20,958)	(11,211)	(57,965)	(19,834)
Loss from operations	(20,958)	(11,211)	(57,965)	(19,834)
Other (expense) income:
Interest income	34	2	84	71
Interest expense	(136)	(146)	(458)	(545)
Other income (expense)		(60)
Total other expense	(102)	(204)	(374)	(474)
Net loss	(21,060)	(11,415)	(58,339)	(20,308)
Deemed dividend related to beneficial conversion feature of Series F convertible preferred stock			(14,840)
Net loss attributable to stockholders	(21,060)	(11,415)	(73,179)	(20,308)
Net loss attributable to preferred stockholders	1,183		3,329
Net loss attributable to common stockholders - basic and diluted	(19,877)	(11,415)	(69,850)	(20,308)
Comprehensive loss applicable to all stockholders	$ (21,060)	$ (11,415)	$ (73,179)	$ (20,308)
Loss per share - basic and diluted	$ (0.59)	$ (1.43)	$ (2.35)	$ (7.18)
Weighted-average common shares - basic and diluted	33,609,212	8,005,142	29,664,548	2,828,757