Basic and Diluted Net Loss Per Share - Computation of Company's Basic and Diluted Net Loss Per Share of Common Stock (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
Numerator
Net loss	$ (10,120)	[1]	$ (11,986)	[1]	$ (13,330)	[1]	$ (12,904)	[1]	$ (13,365)	[1]	$ (8,852)	[1]	$ (13,619)	[1]	$ (10,262)	[1]	$ (48,340)	$ (46,098)	$ (35,390)
Denominator
Weighted-average common shares for basic and diluted net loss per share																	66,818	33,155	22,353
Basic and diluted net loss per share	$ (0.15)		$ (0.18)		$ (0.20)		$ (0.20)		$ (0.22)		$ (0.36)		$ (0.60)		$ (0.45)		$ (0.72)	$ (1.39)	$ (1.58)

[1] In the fourth quarter of 2013, in connection with a debt refinancing transaction, the Company recorded a $1.8 million loss on early extinguishment of debt, which was classified in interest expense. Refer to Note 4 for additional details.