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Investment Securities - Available-for-Sale (Detail) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Amortized Cost $ 156,175 [1] $ 133,611
Available-for-sale securities, Unrealized Gains 2,122 3,282
Available-for-sale securities, Unrealized Losses (2,294) (53)
Available-for-sale securities, Fair Value [2] 156,003 [1] 136,840
U.S. Treasury and Agencies [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Amortized Cost 24,401 21,954
Available-for-sale securities, Unrealized Gains 288 462
Available-for-sale securities, Unrealized Losses (372) (25)
Available-for-sale securities, Fair Value 24,317 22,391
Residential Mortgage-Backed Securities [Member] | Agency [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Amortized Cost 116,629 98,031
Available-for-sale securities, Unrealized Gains 1,230 1,950
Available-for-sale securities, Unrealized Losses (1,683) (13)
Available-for-sale securities, Fair Value 116,176 99,968
Commercial [Member] | Agency [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Amortized Cost 6,254 5,251
Available-for-sale securities, Unrealized Gains 53 170
Available-for-sale securities, Unrealized Losses (221) (15)
Available-for-sale securities, Fair Value 6,086 5,406
Asset-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Amortized Cost 197 [3] 200
Available-for-sale securities, Unrealized Gains 5 5
Available-for-sale securities, Fair Value 202 [3] 205
Obligations of State and Political Subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Amortized Cost 8,687 [4],[5] 8,166
Available-for-sale securities, Unrealized Gains 546 695
Available-for-sale securities, Unrealized Losses (18)  
Available-for-sale securities, Fair Value 9,215 [4],[5] 8,861
Other [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale Securities, Amortized Cost 7 9
Available-for-sale securities, Fair Value $ 7 $ 9
[1] The weighted-average maturity of total available-for-sale investment securities was 3.4 years at December 31, 2020, with a corresponding weighted-average yield of 1.61 percent.
[2] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.
[3] Information related to asset and mortgage-backed securities included above is presented based upon weighted-average maturities that take into account anticipated future prepayments.
[4] Information related to obligations of state and political subdivisions is presented based upon yield to first optional call date if the security is purchased at a premium, and yield to maturity if the security is purchased at par or a discount.
[5] Maturity calculations for obligations of state and political subdivisions are based on the first optional call date for securities with a fair value above par and the contractual maturity date for securities with a fair value equal to or below par.