Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Series A Warrant liability [Member]
Liabilities
Warrant liability	$ 3,582	$ 857
Series B Warrant liability [Member]
Liabilities
Warrant liability	14,180	17,439
Series C Warrant liability [Member]
Liabilities
Warrant liability	2,549
Fair Value, Measurements, Recurring [Member] | Convertible preferred stock warrant liability [Member]
Liabilities
Warrant liability	20,311	18,296
Fair Value, Measurements, Recurring [Member] | Convertible preferred stock warrant liability [Member] | Series A Warrant liability [Member]
Liabilities
Warrant liability	3,582	857
Fair Value, Measurements, Recurring [Member] | Convertible preferred stock warrant liability [Member] | Series B Warrant liability [Member]
Liabilities
Warrant liability	14,180	17,439
Fair Value, Measurements, Recurring [Member] | Convertible preferred stock warrant liability [Member] | Series C Warrant liability [Member]
Liabilities
Warrant liability	2,549
Fair Value, Measurements, Recurring [Member] | Money market fund [Member]
Assets
Cash and cash equivalents	6,472	7,892
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Convertible preferred stock warrant liability [Member]
Liabilities
Warrant liability	3,582	857
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Convertible preferred stock warrant liability [Member] | Series A Warrant liability [Member]
Liabilities
Warrant liability	3,582	857
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Money market fund [Member]
Assets
Cash and cash equivalents	6,472	7,892
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Convertible preferred stock warrant liability [Member]
Liabilities
Warrant liability	16,729	17,439
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Convertible preferred stock warrant liability [Member] | Series B Warrant liability [Member]
Liabilities
Warrant liability	14,180	17,439
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Convertible preferred stock warrant liability [Member] | Series C Warrant liability [Member]
Liabilities
Warrant liability	$ 2,549