Commitments and Contingencies - Summary of Maturities of Sublease Income and Operating Lease Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Sublease Income
2021 (remaining)	$ 1,324
2022	5,357
2023	5,518
2024	5,684
2025	5,854
Thereafter	33,742
Total	57,479
Operating Lease Liabilities
2021 (remaining)	1,768
2022	6,186
2023	5,652
2024	5,796
2025	5,945
Thereafter	34,444
Total	59,791
Present value adjustment	(21,973)
Total	$ 37,818	$ 38,036