Fair Value of Financial Instruments - Summary of Financial Instruments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Series A Warrant Liability [Member]
Liabilities
Warrant liability	$ 289	$ 194	$ 509
Series C Warrant Liability [Member]
Liabilities
Warrant liability	20	86	$ 115
Fair Value, Measurements, Recurring [Member]
Liabilities
Warrant liability	309	280
Fair Value, Measurements, Recurring [Member] | Convertible preferred stock warrant liability [Member] | Series A Warrant Liability [Member]
Liabilities
Warrant liability	289	194
Fair Value, Measurements, Recurring [Member] | Convertible preferred stock warrant liability [Member] | Series C Warrant Liability [Member]
Liabilities
Warrant liability	20	86
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Liabilities
Warrant liability	289	194
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Convertible preferred stock warrant liability [Member] | Series A Warrant Liability [Member]
Liabilities
Warrant liability	289	194
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Liabilities
Warrant liability	20	86
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Convertible preferred stock warrant liability [Member] | Series C Warrant Liability [Member]
Liabilities
Warrant liability	$ 20	$ 86