Variable Interest Entities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014
LIHTC Funds [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Carrying Amount of Assets, Consolidated VIEs	$ 1
Settlement value of third-party interests, reserves	5
Variable Interest Entity, Carrying Amount of Assets, Unconsolidated VIE	55
LIHTC Funds [Member] \| Minimum [Member]
Variable Interest Entity [Line Items]
Settlement value of third-party interests, approximately	0
LIHTC Funds [Member] \| Maximum [Member]
Variable Interest Entity [Line Items]
Settlement value of third-party interests, approximately	4
LIHTC Funds [Member] \| Investments [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Carrying Amount of Assets, Consolidated VIEs	5
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Carrying Amount of Assets, Unconsolidated VIE	1,234
Assets of unconsolidated LIHTC operating partnerships	764
Maximum exposure to loss, unconsolidated VIE	407
Recapture tax claim	110
Additional investments in unconsolidated LIHTC operating partnerships held by consolidated LIHTC	$ 470

Maximum exposure to loss unconsolidated VIE unamortized investments.

No definition available.

Name:	key_MaximumExposureToLossUnconsolidatedVieUnamortizedInvestments
Namespace Prefix:	key_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Variable interest entity assets of unconsolidated operating partnerships.

+ References

No definition available.

+ Details

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

- Definition

Variable interest entity maximum exposure to loss recapture tax credits claim.

+ References

No definition available.

+ Details

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

- Definition

Variable interest entity settlement value of third party interests.

+ References

No definition available.

+ Details

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

- Definition

Variable interest entity settlement value of third party interests including reserves.

+ References

No definition available.

+ Details

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

- Definition

Total assets of operating partnerships that are held by the consolidated guaranteed funds.

+ References

No definition available.

+ Details

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

- Definition

The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685

+ Details

Name:	us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph a

-URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5728-111685

+ Details

Name:	us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=key_VariableInterestSponsorsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: