Income Taxes - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Income Tax Contingency [Line Items]
Total income tax expense (benefit), rate	17.00%	12.90%
Combined federal and state statutory tax rate		23.70%
Income tax benefit (as a percent)	0.90%	5.50%
Federal deferred tax asset	$ 24		$ 222
Valuation allowance	11		$ 11
Key Corp [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits	36
First Niagara Bank, N.A. [Member]
Income Tax Contingency [Line Items]
Allocated bad debt deductions	$ 92