Asset Quality - Changes in Allowance for Loan and Lease Losses by Loan Category (Parenthetical) (Detail) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015
Receivables [Abstract]
Provision/Credit for losses on lending-related commitments	$ 6	$ 10

Financing receivable allowance for credit losses lending related commitments.

No definition available.

Name:	key_FinancingReceivableAllowanceForCreditLossesLendingRelatedCommitments
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration