Contingent Liabilities and Guarantees - Guarantees (Detail) $ in Millions	Sep. 30, 2015 USD ($)
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	$ 15,380
Liability Recorded	188
Written Put Options [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	2,131
Liability Recorded	117
Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	11,498
Liability Recorded	63
Recourse Agreement with FNMA [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	1,747
Liability Recorded	4
Return Guarantee Agreement with LIHTC Investors [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	4
Liability Recorded	$ 4

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsCurrentCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_OpenOptionContractsWrittenTypeAxis=us-gaap_PutOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_FinancialStandbyLetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=key_UnderwritingAndServicingProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=key_ReturnGuaranteeAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: