Variable Interest Entities - Summary of Variable Interest Entities Information (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Total Assets	$ 269	$ 248
Total Liabilities	2	2
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	5,948	5,932
Total Liabilities	2,568	2,569
Maximum Exposure to Loss	1,715	1,740
KCC Indirect Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	16,285	19,659
Total Liabilities	299	376
Maximum Exposure to Loss	$ 108	$ 122

Maximum exposure to loss unconsolidated VIE.

No definition available.

Name:	key_MaximumExposureToLossUnconsolidatedVie
Namespace Prefix:	key_
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_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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph a
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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

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

- Details

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

- Details

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