Fair Value Measurements - Summary of Financial Assets Recognized at Fair Value on Recurring Basis (Detail) - Fair Value on Recurring Basis [Member] - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 354,282	$ 46,917
Marketable securities:
Marketable securities	75,117	227,246
Total	429,399	274,163
U.S. Treasury Securities [Member]
Marketable securities:
Marketable securities	41,562	159,502
Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	28,592	40,278
Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	4,963	18,967
Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities		8,499
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	354,282	46,917
Marketable securities:
Marketable securities	41,562	159,502
Total	395,844	206,419
Level 1 [Member] | U.S. Treasury Securities [Member]
Marketable securities:
Marketable securities	41,562	159,502
Level 2 [Member]
Marketable securities:
Marketable securities	33,555	67,744
Total	33,555	67,744
Level 2 [Member] | Financial Institution Debt Securities [Member]
Marketable securities:
Marketable securities	28,592	40,278
Level 2 [Member] | Corporate Debt Securities [Member]
Marketable securities:
Marketable securities	$ 4,963	18,967
Level 2 [Member] | Other Asset Backed Securities [Member]
Marketable securities:
Marketable securities		$ 8,499