Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Nonrecurring Basis (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Nonrecurring [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Transfer of commercial and consumer loans from held-for-sale to held-to-maturity, current fair value	$ 24	$ 10	$ 11

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