Fair Values of Assets and Liabilities - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	[1]	$ 72,426		$ 69,706
Mortgage loans held for sale		1,885		2,011
Mortgage servicing rights		3,462	$ 3,724	3,377	$ 3,755
Derivative assets netting		(3,485)		(3,666)
Time deposits		4,668		2,818
Derivative liabilities netting		(3,984)		(3,720)
Equity investments without readily determinable fair values		137		133
Equity investments without readily determinable fair values impairment loss cumulative amount		0		5
Equity investments without readily determinable fair values impairment loss		0	$ 0
U.S. Treasury and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		21,518		19,542
Residential mortgage-backed securities | Agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		27,487		26,078
Commercial mortgage-backed securities | Agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7,284		7,343
Commercial mortgage-backed securities | Non-agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6		6
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6,204		6,724
Obligations of state and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		9,787		9,989
Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		140		24
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		72,426		69,706
Mortgage loans held for sale		1,885		2,011
Mortgage servicing rights		3,462		3,377
Derivative assets netting		(3,485)		(3,666)
Derivative assets		2,673		2,865
Other assets		2,635		2,541
Total assets		83,081		80,500
Time deposits		4,668		2,818
Derivative liabilities netting		(3,984)		(3,720)
Derivative liabilities		4,925		4,589
Short-term borrowings and other liabilities		2,259		2,303
Total liabilities		11,852		9,710
Fair Value, Measurements, Recurring | U.S. Treasury and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		21,518		19,542
Fair Value, Measurements, Recurring | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		27,487		26,078
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities | Agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7,284		7,343
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities | Non-agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6		6
Fair Value, Measurements, Recurring | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6,204		6,724
Fair Value, Measurements, Recurring | Obligations of state and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		9,787		9,989
Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		140		24
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		16,817		14,787
Mortgage loans held for sale		0		0
Mortgage servicing rights		0		0
Derivative assets before netting		3		0
Other assets		372		550
Total assets		17,192		15,337
Time deposits		0		0
Derivative liabilities before netting		0		16
Short-term borrowings and other liabilities		436		517
Total liabilities		436		533
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		16,817		14,787
Fair Value, Measurements, Recurring | Level 1 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 1 | Commercial mortgage-backed securities | Agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 1 | Commercial mortgage-backed securities | Non-agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 1 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 1 | Obligations of state and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 1 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		55,609		54,919
Mortgage loans held for sale		1,885		2,011
Mortgage servicing rights		0		0
Derivative assets before netting		4,791		5,078
Other assets		2,263		1,991
Total assets		64,548		63,999
Time deposits		4,668		2,818
Derivative liabilities before netting		5,184		4,955
Short-term borrowings and other liabilities		1,823		1,786
Total liabilities		11,675		9,559
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		4,701		4,755
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		27,487		26,078
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities | Agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7,284		7,343
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities | Non-agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6		6
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6,204		6,724
Fair Value, Measurements, Recurring | Level 2 | Obligations of state and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		9,787		9,989
Fair Value, Measurements, Recurring | Level 2 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		140		24
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Mortgage loans held for sale		0		0
Mortgage servicing rights		3,462		3,377
Derivative assets before netting		1,364		1,453
Other assets		0		0
Total assets		4,826		4,830
Time deposits		0		0
Derivative liabilities before netting		3,725		3,338
Short-term borrowings and other liabilities		0		0
Total liabilities		3,725		3,338
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury and agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 3 | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 3 | Commercial mortgage-backed securities | Agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 3 | Commercial mortgage-backed securities | Non-agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 3 | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 3 | Obligations of state and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		0		0
Fair Value, Measurements, Recurring | Level 3 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		$ 0		$ 0

[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.