Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Income Tax Contingency [Line Items]
Effective tax rate (as a percent)	18.70%	18.80%
Combined federal and state statutory tax rate (as a percent)	24.20%
Valuation allowance	$ 13		$ 15
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	40
Accrued Income And Other Assets
Income Tax Contingency [Line Items]
Net deferred tax assets	$ 1,200		$ 1,600