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, 2021	Mar. 31, 2020
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 2,210	$ 2,546
Net Gains (Losses) Included in Net Income	242	(866)
Sales		1
Issuances	319	201
End of Period Balance	2,787	1,887
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	242	(866)
Purchases	16	5
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	8	9
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		1
End of Period Balance	8	10
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]
Net Change in net derivative asset and liability unrealized gains (losses) relating to assets still held at end of period	(900)	1,726
Beginning of Period Balance	2,326	810
Net Gains (Losses) Included in Net Income	(935)	1,742
Purchases	2	4
Settlements	(237)	(60)
End of Period Balance	1,156	2,496
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	8
End of Period Balance	7	8
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
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		1
End of Period Balance	$ 1	2
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		$ 1