Fair Value Disclosures - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Assets:
Assets, fair value disclosure	$ 656,864	$ 980,192
Money Market Funds
Assets:
Assets, fair value disclosure	170,171	64,361
US government and government agency notes
Assets:
Assets, fair value disclosure	195,184	621,400
Corporate debt securities
Assets:
Assets, fair value disclosure	176,808	162,906
Commercial Paper
Assets:
Assets, fair value disclosure	114,701	131,525
Quoted Prices in Active Markets (Level 1)
Assets:
Assets, fair value disclosure	365,355	685,761
Quoted Prices in Active Markets (Level 1) | Money Market Funds
Assets:
Assets, fair value disclosure	170,171	64,361
Quoted Prices in Active Markets (Level 1) | US government and government agency notes
Assets:
Assets, fair value disclosure	195,184	621,400
Quoted Prices in Active Markets (Level 1) | Corporate debt securities
Assets:
Assets, fair value disclosure	0	0
Quoted Prices in Active Markets (Level 1) | Commercial Paper
Assets:
Assets, fair value disclosure	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Assets, fair value disclosure	291,509	294,431
Significant Other Observable Inputs (Level 2) | Money Market Funds
Assets:
Assets, fair value disclosure	0	0
Significant Other Observable Inputs (Level 2) | US government and government agency notes
Assets:
Assets, fair value disclosure	0	0
Significant Other Observable Inputs (Level 2) | Corporate debt securities
Assets:
Assets, fair value disclosure	176,808	162,906
Significant Other Observable Inputs (Level 2) | Commercial Paper
Assets:
Assets, fair value disclosure	114,701	131,525
Significant Unobservable Inputs (Level 3)
Assets:
Assets, fair value disclosure	0	0
Significant Unobservable Inputs (Level 3) | Money Market Funds
Assets:
Assets, fair value disclosure	0	0
Significant Unobservable Inputs (Level 3) | US government and government agency notes
Assets:
Assets, fair value disclosure	0	0
Significant Unobservable Inputs (Level 3) | Corporate debt securities
Assets:
Assets, fair value disclosure	0	0
Significant Unobservable Inputs (Level 3) | Commercial Paper
Assets:
Assets, fair value disclosure	$ 0	$ 0