Fair Value Measurements - Assets Measured at Fair Value on Nonrecurring Basis (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Transfer of commercial and consumer loans from held-for-sale to held-to-maturity, current fair value	$ 2	$ 17	$ 17

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

No definition available.

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