Fair Value of Financial Assets and Liabilities - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 126,539	$ 5,708
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		406
Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	126,047	5,684
Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	492	24
Derivative Liabilities [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		371
Warrants to Purchase Redeemable Convertible Preferred Stock [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		35
Level 1 [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	126,539	5,708
Level 1 [Member] | Money Market Funds [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	126,047	5,684
Level 1 [Member] | Restricted Cash [Member]
Assets Fair Value Disclosure
Assets Fair Value Disclosure	$ 492	24
Level 3 [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		406
Level 3 [Member] | Derivative Liabilities [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		371
Level 3 [Member] | Warrants to Purchase Redeemable Convertible Preferred Stock [Member]
Liabilities Fair Value Disclosure
Liabilities Fair Value Disclosure		$ 35