Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Income Tax Contingency [Line Items]
Effective tax rate (as a percent)	18.50%	16.70%
Combined federal and state statutory tax rate (as a percent)	23.90%
Valuation allowance	$ 12		$ 12
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	$ 1,900		$ 1,800