Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Details)	Sep. 30, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ 1,601,000,000	$ 1,962,000,000
Mortgage servicing assets excluded from OREO	0	67,000,000
Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	1,601,000,000	1,962,000,000
Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	69,000,000	28,000,000
Level 3
Fair Value, Option, Quantitative Disclosures [Line Items]
Insignificant level 3 assets, net of liabilities	(4,000,000)	9,000,000
Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	(8,000,000)	39,000,000
Level 3 \| Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	69,000,000	28,000,000
Valuation, Market Comparable Pricing \| Level 3 \| Recurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential)	$ 9,000,000	$ 11,000,000
Valuation, Market Comparable Pricing \| Level 3 \| Recurring \| Measurement Input, Comparability Adjustment \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.5347	0.6450
Valuation, Market Comparable Pricing \| Level 3 \| Recurring \| Measurement Input, Comparability Adjustment \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.8589	0.9730
Valuation, Market Comparable Pricing \| Level 3 \| Recurring \| Measurement Input, Comparability Adjustment \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Loans, net of unearned income (residential), measurement input	0.8088	0.9424
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring \| Interest rate
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets	$ (8,000,000)	$ 33,000,000
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring \| Interest rate \| Measurement Input, Default Rate \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.0002	0.0002
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring \| Interest rate \| Measurement Input, Default Rate \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring \| Interest rate \| Measurement Input, Default Rate \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.1490	0.0888
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring \| Interest rate \| Measurement Input, Loss Given Default \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0	0
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring \| Interest rate \| Measurement Input, Loss Given Default \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	1	1
Valuation Technique, Discounted Cash Flow \| Level 3 \| Recurring \| Interest rate \| Measurement Input, Loss Given Default \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Derivative assets, measurement input	0.499	0.500
Valuation, Fair Value Of Underlying Collateral \| Level 3 \| Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	$ 69,000,000	$ 28,000,000
Valuation, Fair Value Of Underlying Collateral \| Level 3 \| Nonrecurring \| Measurement Input, Discount Rate \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0	0
Valuation, Fair Value Of Underlying Collateral \| Level 3 \| Nonrecurring \| Measurement Input, Discount Rate \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.8500	0.1000
Valuation, Fair Value Of Underlying Collateral \| Level 3 \| Nonrecurring \| Measurement Input, Discount Rate \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans, measurement input	0.2600	0.0800
Valuation, Appraised Value \| Level 3 \| Nonrecurring
Fair Value, Option, Quantitative Disclosures [Line Items]
OREO and other Level 3 assets	$ 17,000,000	$ 13,000,000

Impaired Loan, Measurement Input

No definition available.

Name:	key_ImpairedLoanMeasurementInput
Namespace Prefix:	key_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Impaired loans fair value disclosure.

+ References

No definition available.

+ Details

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

- Definition

Insignificant Assets (Liabilities), Net, Fair Value Disclosure

+ References

No definition available.

+ Details

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

- Definition

Loans Held-For-Investment, Fair Value Disclosure

+ References

No definition available.

+ Details

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

- Definition

Loans Held-For-Investment, Measurement Input

+ References

No definition available.

+ Details

Name:	key_LoansHeldForInvestmentMeasurementInput
Namespace Prefix:	key_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Mortgage Servicing Asset, Excluded From Other Real Estate

+ References

No definition available.

+ Details

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

- Definition

Value of input used to measure derivative asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

+ Details

Name:	us-gaap_DerivativeAssetMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 22
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226052-175313

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI https://asc.fasb.org/extlink&oid=123594938&loc=d3e13433-108611

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_FairValueOptionQuantitativeDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(4))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputComparabilityAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_ValuationTechniqueDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDefaultRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=key_MeasurementInputLossGivenDefaultMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=key_ValuationFairValueOfUnderlyingCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=key_ValuationAppraisedValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: