Quarterly Financial Data (Unaudited) - Summary of Quarterly Results (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
Income Statement [Abstract]
Revenue	$ 8,912	$ 8,036	$ 14,469	$ 14,268	$ 11,536	$ 11,115	$ 10,555	$ 8,307	$ 45,685	$ 41,513	$ 67,243
Gross loss	(5,459)	(9,460)	(7,274)	(7,494)	(5,811)	(6,318)	(6,417)	(3,375)	(29,687)	(21,921)	(40)
Loss from operations	(8,637)	(13,872)	(10,179)	(10,925)	(9,025)	(9,595)	(9,968)	(6,296)	(43,613)	(34,884)	(13,036)
Loss before income taxes	(9,032)	(14,011)	(9,984)	(10,888)	(9,106)	(9,814)	(10,173)	(6,418)	(43,915)	(35,511)	(12,586)
Net loss	$ (9,402)	$ (13,713)	$ (9,984)	$ (10,894)	$ (15,241)	$ (5,840)	$ (5,894)	$ (3,376)	$ (43,993)	$ (30,351)	$ (5,538)
Basic and diluted loss per common share	$ (0.36)	$ (0.53)	$ (0.39)	$ (0.43)	$ (0.67)	$ (0.26)	$ (0.26)	$ (0.15)	$ (1.70)	$ (1.35)
Weighted average common shares outstanding used in computing net loss per common share, basic and diluted:	26,127,026	26,110,651	25,706,797	25,317,147	22,599,258	22,578,608	22,560,603	22,550,378	25,815,405	22,572,212