Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 14,636	$ 33,900	$ 8,411	$ 8,411	$ 6,744	$ 6,268	$ 6,268	$ 6,268	$ 65,358	$ 25,548	$ 25,106
Income (loss) from operations	(26,727)	3,208	(18,330)	(12,284)	(12,242)	(11,069)	(8,526)	(7,046)	(54,133)	(38,883)	(22,995)
Net income (loss)	(26,664)	3,704	(18,296)	(12,248)	(12,382)	(11,177)	(8,620)	(7,228)	(53,504)	(39,407)	(20,102)
Net income (loss) applicable to common stockholders	$ (26,664)	$ 3,704	$ (18,296)	$ (12,248)	$ (12,382)	$ (11,744)	$ (10,418)	$ (9,025)	$ (53,504)	$ (43,569)	$ (27,292)
Net income (loss) per share applicable to common stockholders - basic	$ (0.76)	$ 0.11	$ (0.54)	$ (0.39)
Net loss per share applicable to common stockholders - basic and diluted					$ (0.40)	$ (0.52)	$ (2.80)	$ (2.47)	$ (1.59)	$ (2.83)	$ (8.02)
Net income (loss) per share applicable to common stockholders - diluted	$ (0.76)	$ 0.10	$ (0.54)	$ (0.39)