Securities - Available for Sale Securities (Unrealized Loss Position) (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Mar. 31, 2012
Available For Sale Securities [Abstract]
Gross unrealized losses	$ 1	$ 1

Securities Continuous Unrealized Loss Position Aggregate Losses

No definition available.

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