Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Additional net investment tax credits		$ 14
Percent reduction to income tax provision		2.80%
Tax reform, income tax expense (benefit)					$ 161
Tax reform, low-income housing investments, income tax expense (benefit)				$ 14
Tax reform, change in tax rate, income tax expense (benefit)				$ 147
Tax reform, change in enacted tax rate, percent				7.60%
Tax reform, measurement period adjustment	$ 7
Tax reform, total impact				$ 168
Tax reform, reclassification from AOCI to retained earnings, tax effect	$ 141
Total income tax expense (benefit), rate	15.50%		26.90%
Combined federal and state statutory tax rate				23.70%	37.20%
Income tax benefit (as a percent)	2.20%		2.20%
Federal deferred tax asset	$ 331			$ 331	$ 320
Valuation allowance	11			11	$ 15
Key Corp [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits	46			46
First Niagara Bank, N.A. [Member]
Income Tax Contingency [Line Items]
Allocated bad debt deductions	$ 92			$ 92