Netting Arrangements for Certain Financial Instruments and Securities Financing Activities - Information on Company's Accounting Netting Adjustments and Items Not Offset in Consolidated Balance Sheet Assets But Available for Offset in Event of Default (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Offsetting [Abstract]
Cash collateral netted against derivative assets	$ 750	$ 898
Derivative assets not subject to netting arrangements	$ 93	$ 257