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Fair Values of Assets and Liabilities - Changes in Fair Value for All Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Mortgage Servicing Rights [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning of Period Balance $ 2,546 $ 2,791 $ 2,645
Net Gains (Losses) Included in Net Income (1,403) (829) (232)
Purchases 34 20 8
Sales 3 5 (27)
Issuances 1,030 559 397
End of Period Balance 2,210 2,546 2,791
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period (1,403) (829) (232)
Available-for-Sale Securities [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning of Period Balance 9    
Principal Payments (1)    
Transfers into Level 3   9  
End of Period Balance 8 9  
Derivative [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning of Period Balance 810 80 107
Net Gains (Losses) Included in Net Income 2,922 769 21
Purchases 247 142 13
Sales (3) (9) (41)
Settlements (1,650) (172) (20)
End of Period Balance 2,326 810 80
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period 1,649 782 $ 34
Asset-Backed Securities [Member] | Available-for-Sale Securities [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning of Period Balance 8    
Principal Payments (1)    
Transfers into Level 3   8  
End of Period Balance 7 8  
Asset-Backed Securities [Member] | Available-for-Sale Securities [Member] | Obligations of State and Political Subdivisions [Member]      
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]      
Beginning of Period Balance 1    
Transfers into Level 3   1  
End of Period Balance $ 1 $ 1