Fair Value Measurements - Summary of Financial Assets Recognized at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and restricted cash equivalents	$ 335,916	$ 163,805
Marketable securities:
Marketable securities	813,982	437,351
Total	1,149,898	601,156
U.S. Treasury and Other Government Securities [Member]
Marketable securities:
Marketable securities	251,242	245,668
Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	394,998	138,443
Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	32,090	41,766
Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities	135,652	11,474
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents and restricted cash equivalents	335,916	163,805
Marketable securities:
Marketable securities	220,670	241,664
Total	556,586	405,469
Level 1 [Member] | U.S. Treasury and Other Government Securities [Member]
Marketable securities:
Marketable securities	220,670	241,664
Level 2 [Member]
Marketable securities:
Marketable securities	593,312	195,687
Total	593,312	195,687
Level 2 [Member] | U.S. Treasury and Other Government Securities [Member]
Marketable securities:
Marketable securities	30,572	4,004
Level 2 [Member] | Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	394,998	138,443
Level 2 [Member] | Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	32,090	41,766
Level 2 [Member] | Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities	$ 135,652	$ 11,474