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	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 2,787	$ 1,887	$ 2,210	$ 2,546
Net Gains (Losses) Included in Net Income	(379)	(241)	(137)	(1,107)
Purchases	11	3	27	8
Sales	1	1	1	2
Issuances	293	190	612	391
End of Period Balance	2,713	1,840	2,713	1,840
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(379)	(241)	(137)	(1,107)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	8	10	8	9
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	1	(1)	1
Principal Payments		(1)		(1)
End of Period Balance	9	8	9	8
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	1	(2)	1
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	2	1	1
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		(1)
End of Period Balance	1	1	1	1
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		(1)
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	1,156	2,496	2,326	810
Net Gains (Losses) Included in Net Income	556	732	(379)	2,474
Purchases	58	91	60	95
Sales	(1)	(1)	(1)	(1)
Settlements	(269)	(477)	(506)	(537)
End of Period Balance	1,500	2,841	1,500	2,841
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	412	640	(496)	2,066
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	7	8	7	8
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	1		1
Principal Payments		(1)		(1)
End of Period Balance	8	7	8	$ 7
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ 1	$ (1)	$ 1