Netting Arrangements for Certain Financial Instruments and Securities Financing Activities - Summary of Maturities by Category of Collateral Pledged for Repurchase Agreements and Securities Loaned Transactions (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	$ 1,430	$ 1,165
Securities loaned	218	50
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,648	1,215
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	1,430	1,165
Securities loaned	218	50
Gross amount of recognized liabilities for repurchase agreements and securities loaned	1,648	1,215
U.S. Treasury and Agencies [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	472	289
U.S. Treasury and Agencies [Member] | Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	472	289
Residential Mortgage-Backed Securities [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	398	266
Residential Mortgage-Backed Securities [Member] | Overnight and Continuous [Member] | Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	398	266
Corporate Debt Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	560	610
Securities loaned	218	50
Corporate Debt Securities [Member] | Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Repurchase agreements	560	610
Securities loaned	$ 218	$ 50