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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 3,633	$ 3,707	$ 3,755	$ 2,953
Net Gains (Losses) Included in Net Income	134	27	(37)	435
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	1	1	3	7
Sales	(292)	0	(440)	1
Principal Payments	0	0	0	0
Issuances	106	134	301	473
Settlements	0	0	0	0
End of Period Balance	3,582	3,869	3,582	3,869
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	134	27	(37)	435
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance		1	1	8
Net Gains (Losses) Included in Net Income		0	0	0
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		0	0	(3)
Purchases		0	0	0
Sales		0	0	(4)
Principal Payments		0	(1)	0
Issuances		0	0	0
Settlements		0	0	0
End of Period Balance	0	1	0	1
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		0	0	0
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
Net Gains (Losses) Included in Net Income		0	0	0
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		0	0	0
Purchases		0	0	0
Sales		0	0	0
Principal Payments		0	(1)	0
Issuances		0	0	0
Settlements		0	0	0
End of Period Balance	0	1	0	1
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		0	0	0
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	(3,419)	(2,175)	(3,199)	799
Net Gains (Losses) Included in Net Income	(1,315)	(2,398)	(3,558)	(5,759)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	25	259	430	351
Sales	(9)	(29)	(28)	(30)
Principal Payments	0	0	0	0
Issuances	0	11	0	11
Settlements	962	456	2,599	752
End of Period Balance	(3,756)	(3,876)	(3,756)	(3,876)
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ (693)	(1,978)	$ (1,925)	(4,240)
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
Net Gains (Losses) Included in Net Income				0
Net Gains (Losses) Included in Other Comprehensive Income (Loss)				(3)
Purchases				0
Sales				(4)
Principal Payments				0
Issuances				0
Settlements				0
End of Period Balance		$ 0		0
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period				$ 0