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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 1,887	$ 2,656	$ 2,546	$ 2,791
Net Gains (Losses) Included in Net Income	(241)	(331)	(1,107)	(545)
Purchases	3	6	8	7
Sales	1		2
Issuances	190	127	391	205
End of Period Balance	1,840	2,458	1,840	2,458
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(241)	(331)	(1,107)	(545)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	10		9
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(1)
Principal Payments	(1)		(1)
End of Period Balance	8		8
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(2)
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	2		1
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(1)
End of Period Balance	1		1
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	2,496	455	810	80
Net Gains (Losses) Included in Net Income	732	568	2,474	931
Purchases	91	53	95	54
Sales	(1)	(1)	(1)	(8)
Settlements	(477)	(30)	(537)	(12)
End of Period Balance	2,841	1,045	2,841	1,045
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	640	$ 662	2,066	$ 1,019
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
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	1
Principal Payments	(1)		(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	2		1
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(1)
End of Period Balance	1		$ 1
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ (1)