Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Income Tax Contingency [Line Items]
Total income tax expense (benefit), rate	20.40%	12.50%
Combined federal and state statutory tax rate			23.70%
Valuation allowance	$ 12		$ 12	$ 0
Accrued Income And Other Assets
Income Tax Contingency [Line Items]
Net deferred tax assets	41		41
Accrued Liabilities and Other Liabilities
Income Tax Contingency [Line Items]
Net deferred tax liability				$ 100
Key Corp
Income Tax Contingency [Line Items]
Unrecognized tax benefits	55		55
First Niagara Bank, N.A.
Income Tax Contingency [Line Items]
Allocated bad debt deductions	$ 92		$ 92