Fair Value of Financial Assets and Liabilities - Summary of Changes in Fair Value of Preferred Stock Warrant Liability for Fair Value Determined by Level 3 Inputs (Detail) - Level 3 [Member] - Warrants to Purchase Redeemable Convertible Preferred Stock [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 35
Initial fair value of warrant to purchase redeemable convertible preferred stock		$ 18
Change in fair value	162	17
Conversion of preferred stock warrant to common stock warrant	$ (197)
Ending balance		$ 35