Significant Accounting Policies - Summary of Derivative Financial Instruments for Non-employee Stock Options (Detail) - Fair Value, Measurements, Recurring - Black Scholes Model - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Non-employee Stock Options
Liabilities:
Fair value, liabilities	$ 2,637	$ 1,617
Warrants
Liabilities:
Fair value, liabilities	6,941	2,857
Level 3 | Non-employee Stock Options
Liabilities:
Fair value, liabilities	2,637	1,617
Level 3 | Warrants
Liabilities:
Fair value, liabilities	$ 6,941	$ 2,857