Commitments and Contingencies - Schedule of Future Minimum Lease Payments under Non-cancellable Operating Leases, Reconciliation of Undiscounted Cash Flows to the Operating Lease Liability (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Operating Lease Liabilities, Payments Due [Abstract]
2021 (Excluding the nine months ended September 30, 2021)	$ 485
2022	1,957
2023	1,992
Total future minimum lease payments, undiscounted	4,434
Less imputed interest	(345)
Operating lease liabilities	4,089
Operating lease liabilities reported as of September 30, 2021:
Operating lease liabilities - current	1,716	$ 1,602
Operating lease liabilities - non-current	2,373	$ 3,673
Total	$ 4,089