Variable Interest Entities - Variable Interest Entities Information (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
LIHTC Funds [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Carrying Amount of Assets, Consolidated VIEs	$ 1,000,000	$ 15,000,000	$ 1,000,000
Variable Interest Entity, Carrying Amount of Liabilities, Consolidated VIEs	2,000,000	15,000,000	1,000,000
Variable Interest Entity, Carrying Amount of Assets, Unconsolidated VIEs	55,000,000	97,000,000	55,000,000
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Carrying Amount of Assets, Unconsolidated VIEs	1,355,000,000	1,595,000,000	1,234,000,000
Variable Interest Entity, Carrying Amount of Liabilities, Unconsolidated VIEs	4,000,000	5,000,000	4,000,000
Maximum Exposure to Loss, Unconsolidated VIEs	509,000,000	492,000,000	521,000,000
Education Loan Securitization Trusts [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Carrying Amount of Assets, Consolidated VIEs		1,912,000,000
Variable Interest Entity, Carrying Amount of Liabilities, Consolidated VIEs		$ 1,816,000,000

Maximum exposure to loss unconsolidated VIE.

No definition available.

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

- 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

+ Details

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

- Details

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- 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

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Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount of the liabilities 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

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
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- Details

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- Details

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