Quarterly Financial Data - Schedule of Unaudited Consolidated Financial Statements (Detail) (USD $) In Thousands, except 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]
Revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Operating expenses	10,664	14,361	5,986	4,521	3,761	3,524	2,975	3,520	35,532	13,780	13,791
Net loss attributable to common stockholders	$ (10,931)	$ (14,689)	$ (6,443)	$ (4,507)	$ (3,899)	$ (3,557)	$ (3,190)	$ (3,594)	$ (36,570)	$ (14,240)	$ (13,947)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.48)	$ (0.65)	$ (2.96)	$ (6.18)	$ (5.35)	$ (4.88)	$ (4.37)	$ (4.94)	$ (3.00)	$ (19.53)	$ (19.65)
Weighted average common shares outstanding, basic and diluted	22,783,817	22,707,012	2,178,465	729,391	729,391	729,391	729,391	727,763	12,189,155	729,001	709,678