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
	Mar. 31, 2020	Mar. 31, 2019
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 2,546	$ 2,791
Net Gains (Losses) Included in Net Income	(866)	(214)
Purchases	5	1
Sales	1
Issuances	201	78
End of Period Balance	1,887	2,656
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(866)	(214)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	9
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	1
End of Period Balance	10
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
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	1
End of Period Balance	2
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	1
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	810	80
Net Gains (Losses) Included in Net Income	1,742	363
Purchases	4	1
Sales		(7)
Settlements	(60)	18
End of Period Balance	2,496	455
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	1,726	$ 424
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	7
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(1)
End of Period Balance	6
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(1)
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
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	1
End of Period Balance	2
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ 1