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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 2,713	$ 1,840	$ 2,210	$ 2,546
Net Gains (Losses) Included in Net Income	(96)	(192)	(233)	(1,299)
Purchases	9	8	36	16
Sales		1	1	3
Issuances	284	321	896	712
End of Period Balance	2,910	1,978	2,910	1,978
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(96)	(192)	(233)	(1,299)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	9	8	8	9
Net Gains (Losses) Included in Other Comprehensive Income (Loss)			1
Principal Payments				(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
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	1,500	2,841	2,326	810
Net Gains (Losses) Included in Net Income	(225)	211	(604)	2,685
Purchases	106	152	166	247
Sales	(1)	(1)	(2)	(2)
Settlements	(262)	(579)	(768)	(1,116)
End of Period Balance	1,118	2,624	1,118	2,624
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(203)	228	(761)	1,888
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	8
Net Gains (Losses) Included in Other Comprehensive Income (Loss)			1
Principal Payments				(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