Asset Quality - Post-Modification Outstanding Recorded Investment by Concession Type for Our Commercial Accruing and Nonaccruing TDRs (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Receivables [Abstract]
Additional funds to borrowers related to lending related commitments	$ 8	$ 5	$ 1

Significant commitments to lend additional funds to borrowers with loans on nonperforming status.

No definition available.

Name:	key_SignificantCommitmentsToLendAdditionalFundsToBorrowersWithLoansOnNonperformingStatus
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant