Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Income Tax Contingency [Line Items]
Effective tax rate (as a percent)	17.80%	18.70%
Combined federal and state statutory tax rate (as a percent)	23.90%
Valuation allowance	$ 12		$ 11
First Niagara Bank, N.A.
Income Tax Contingency [Line Items]
Allocated bad debt deductions	92
Key Corp
Income Tax Contingency [Line Items]
Unrecognized tax benefits	45
Accrued Income And Other Assets
Income Tax Contingency [Line Items]
Net deferred tax assets	$ 2,200		$ 2,000