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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 3,432	$ 2,787	$ 2,953	$ 2,210
Net Gains (Losses) Included in Net Income	170	(379)	408	(137)
Purchases	3	11	6	27
Sales		1	1	1
Issuances	102	293	339	612
End of Period Balance	3,707	2,713	3,707	2,713
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	170	(379)	408	(137)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	8	8	8	8
Net Gains (Losses) Included in Net Income	(3)		(3)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		1		1
Sales	(4)		(4)
End of Period Balance	1	9	1	9
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		1		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	1	1	1
End of Period Balance	1	1	1	1
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	(1,011)	1,156	799	2,326
Net Gains (Losses) Included in Net Income	(1,494)	556	(3,361)	(379)
Purchases	81	58	92	60
Sales		(1)	(1)	(1)
Settlements	249	(269)	296	(506)
End of Period Balance	(2,175)	1,500	(2,175)	1,500
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(1,259)	412	(2,739)	(496)
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	7	7	7
Net Gains (Losses) Included in Net Income	(3)		(3)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		1		1
Sales	$ (4)		$ (4)
End of Period Balance		8		8
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		$ 1		$ 1