Fair Value of Financial Assets and Liabilities - Summary of Aggregate Fair Values of Warrant Liability and Derivative Liability and Preferred Stock Repurchase Liability (Detail) - Level 3 [Member] $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($)
Preferred Stock Warrant Liability [Member]
Fair Value Assets Measured On Recurring and Nonrecurring Basis [Line Items]
Balance as of December 31, 2018	$ 4,947
Change in fair value	288
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	(5,235)
Balance as of March 31, 2019	0
Derivative Liability [Member]
Fair Value Assets Measured On Recurring and Nonrecurring Basis [Line Items]
Balance as of December 31, 2018	201
Change in fair value	(183)
Balance as of March 31, 2019	$ 18

Fair Value of Financial Assets and Liabilities - Summary of Aggregate Fair Values of Warrant Liability and Derivative Liability and Preferred Stock Repurchase Liability (Detail) - Level 3 [Member]
$ in Thousands

3 Months Ended

Mar. 31, 2019

USD ($)

Preferred Stock Warrant Liability [Member]

Fair Value Assets Measured On Recurring and Nonrecurring Basis [Line Items]

Balance as of December 31, 2018

$ 4,947

Change in fair value

288

Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger

(5,235)

Balance as of March 31, 2019