Trust Preferred Securities Issued by Unconsolidated Subsidiaries - Additional Information (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Trust Preferred Securities [Abstract]
Percentage of trust preferred securities issued by the KeyCorp of Tier 1 capital	8.00%
Trust preferred securities issued by the KeyCorp of Tier 1 capital	$ 85

Percentage of capital securities issued by capital trusts of tier one capital.

No definition available.

Name:	key_PercentageOfCapitalSecuritiesIssuedByCapitalTrustsOfTierOneCapital
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant