Asset Quality - Nonperforming Assets and Past Due Loans (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Mar. 31, 2013
Disclosure Loans Classified As Nonperforming [Abstract]
Performing home equity second liens including nonperforming loans	$ 72
Purchased credit impaired loans	23	22
Restructured loans	$ 3	$ 4

Purchased Credit Impaired Loans Outstanding Principal Balance

No definition available.

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