Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Additional net investment tax credits	$ 13.6
Percent reduction to income tax provision	2.80%
Tax reform, income tax expense (benefit)	$ 161.0
Tax reform, low-income housing investments, income tax expense (benefit)	14.0
Tax reform, change in tax rate, income tax expense (benefit)	$ 147.0
Tax reform, change in enacted tax rate, percent	7.60%
Tax reform, reclassification from AOCI to retained earnings, tax effect	$ 141.0
Total income tax expense (benefit), rate	12.90%	22.40%
Combined federal and state statutory tax rate	23.70%		37.20%
Income tax benefit (as a percent)	5.50%	4.70%
Federal deferred tax asset	$ 363.0		$ 320.0
Valuation allowance	15.0
Key Corp [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits	39.0
First Niagara Bank, N.A. [Member]
Income Tax Contingency [Line Items]
Allocated bad debt deductions	$ 92.0