Fair Value Measurements - Assets Measured at Fair Value on Nonrecurring Basis (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Fair Value Disclosures [Abstract]
Transfer of commercial and consumer loans from held-for-sale to held-to-maturity, current fair value	$ 1

Transfer of commercial and consumer loans from held-for-sale to held-to-maturity, current fair value.

No definition available.

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