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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 1,840	$ 2,458	$ 2,546	$ 2,791
Net Gains (Losses) Included in Net Income	(192)	(340)	(1,299)	(885)
Purchases	8	6	16	13
Sales	1	4	3	4
Issuances	321	168	712	373
End of Period Balance	1,978	2,296	1,978	2,296
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(192)	(340)	(1,299)	(885)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	8		9
Principal Payments			(1)
End of Period Balance	8		8
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	2,841	1,045	810	80
Net Gains (Losses) Included in Net Income	211	313	2,685	1,244
Purchases	152	1	247	55
Sales	(1)	(1)	(2)	(9)
Settlements	(579)	(72)	(1,116)	(84)
End of Period Balance	2,624	1,286	2,624	1,286
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	228	$ 322	1,888	$ 1,256
Asset-Backed Securities [Member] | Available-for-Sale Securities [Member] | Other Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	6		7
Principal Payments			(1)
End of Period Balance	6		6
Asset-Backed Securities [Member] | Available-for-Sale Securities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	1		1
End of Period Balance	1		1
Asset-Backed Securities [Member] | 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
End of Period Balance	$ 1		$ 1