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		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 3,707	$ 2,713	$ 2,953	$ 2,210
Net Gains (Losses) Included in Net Income	27	(96)	435	(233)
Purchases	1	9	7	36
Sales		0	1	1
Issuances	134	284	473	896
End of Period Balance	3,869	2,910	3,869	2,910
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	27	(96)	435	(233)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	1	9	8	8
Net Gains (Losses) Included in Net Income	0		0
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		0	(3)	1
Sales	0		(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		0		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	(2,175)	1,500	799	2,326
Net Gains (Losses) Included in Net Income	(2,398)	(225)	(5,759)	(604)
Purchases	259	106	351	166
Sales	(29)	(1)	(30)	(2)
Issuances	11		11
Settlements	456	(262)	752	(768)
End of Period Balance	(3,876)	1,118	(3,876)	1,118
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ (1,978)	(203)	(4,240)	(761)
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	7	7
Net Gains (Losses) Included in Net Income			0
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		0	(3)	1
Sales			$ (4)
End of Period Balance		8		8
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		$ 0		$ 1