Selected Quarterly Financial Data (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statements of Operations and Comprehensive Loss
Net loss	$ (33,240,000)	$ (28,264,000)	$ (22,965,000)	$ (17,057,000)	$ (17,962,000)	$ (17,420,000)	$ (12,609,000)	$ (14,488,000)	$ (101,526,000)	$ (62,479,000)	$ (60,690,000)
Net loss applicable to common stock	(33,240,000)	(28,264,000)	(22,965,000)	(17,057,000)	(17,962,000)	(17,420,000)	(16,640,000)	(19,457,000)	$ (101,526,000)	$ (71,479,000)	$ (78,161,000)
Net loss per share - basic and diluted	$ (0.77)	$ (0.68)	$ (0.61)	$ (0.47)	$ (0.55)	$ (0.59)	$ (2.22)	$ (50.45)
Weighted-average common shares outstanding - basic and diluted	42,924,137	41,331,612	37,895,651	36,268,975	32,678,459	29,746,426	7,500,148	385,664	39,643,099	17,699,487	383,310