Derivatives and Hedging Activities - Schedule of Pre-Tax Net Gains (Losses) on Derivatives Not Designated as Hedging Instruments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	$ 31	$ 9	$ 98	$ 30
Interest rate
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	13	13	21	27
Foreign exchange
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	12	15	25	27
Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	1	2	3	4
Credit
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	(10)	(7)	(18)	(19)
Other
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	15	(14)	67	(9)
Location, Statement of Income, Balance [Axis]: us-gaap:NoninterestIncomeOther
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	7	(21)	49	(22)
Location, Statement of Income, Balance [Axis]: us-gaap:NoninterestIncomeOther \| Interest rate
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	6
Location, Statement of Income, Balance [Axis]: us-gaap:NoninterestIncomeOther \| Foreign exchange
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: us-gaap:NoninterestIncomeOther \| Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: us-gaap:NoninterestIncomeOther \| Credit
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	(10)	(7)	(18)	(19)
Location, Statement of Income, Balance [Axis]: us-gaap:NoninterestIncomeOther \| Other
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	17	(14)	67	(9)
Location, Statement of Income, Balance [Axis]: key:BankingServicesIncome
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	26	30	49	52
Location, Statement of Income, Balance [Axis]: key:BankingServicesIncome \| Interest rate
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	13	13	21	21
Location, Statement of Income, Balance [Axis]: key:BankingServicesIncome \| Foreign exchange
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	12	15	25	27
Location, Statement of Income, Balance [Axis]: key:BankingServicesIncome \| Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	1	2	3	4
Location, Statement of Income, Balance [Axis]: key:BankingServicesIncome \| Credit
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: key:BankingServicesIncome \| Other
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: key:ConsumerMortgageIncome
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	(2)	0	0	0
Location, Statement of Income, Balance [Axis]: key:ConsumerMortgageIncome \| Interest rate
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: key:ConsumerMortgageIncome \| Foreign exchange
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: key:ConsumerMortgageIncome \| Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: key:ConsumerMortgageIncome \| Credit
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	0	0	0	0
Location, Statement of Income, Balance [Axis]: key:ConsumerMortgageIncome \| Other
Derivative Instruments, Gain (Loss) [Line Items]
Total net gains (losses)	$ (2)	$ 0	$ 0	$ 0