Mortgage Servicing Assets - Schedule of Range and Weighted-Average of Significant Unobservable Inputs (Details) - USD ($)	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022
Agency commercial mortgage-backed securities \| Minimum
Servicing Assets at Fair Value [Line Items]
Expected defaults	1.00%	0.97%
Residual cash flows discount rate	7.42%	8.54%
Escrow earn rate	5.67%	5.09%
Loan assumption rate	0.00%	0.00%
Agency commercial mortgage-backed securities \| Maximum
Servicing Assets at Fair Value [Line Items]
Expected defaults	2.00%	2.00%
Residual cash flows discount rate	10.56%	10.02%
Escrow earn rate	5.72%	5.21%
Loan assumption rate	2.15%	1.41%
Agency commercial mortgage-backed securities \| Weighted-Average
Servicing Assets at Fair Value [Line Items]
Expected defaults	1.01%	1.07%
Residual cash flows discount rate	10.17%	9.48%
Escrow earn rate	5.67%	5.17%
Loan assumption rate	1.97%	1.12%
Agency residential mortgage-backed securities \| Minimum
Servicing Assets at Fair Value [Line Items]
Residual cash flows discount rate	6.50%	7.50%
Prepayment speed	6.27%	6.10%
Servicing cost	$ 70.00	$ 62.00
Agency residential mortgage-backed securities \| Maximum
Servicing Assets at Fair Value [Line Items]
Residual cash flows discount rate	8.75%	8.50%
Prepayment speed	44.47%	41.34%
Servicing cost	$ 3,582	$ 4,375
Agency residential mortgage-backed securities \| Weighted-Average
Servicing Assets at Fair Value [Line Items]
Residual cash flows discount rate	6.59%	7.53%
Prepayment speed	7.70%	7.20%
Servicing cost	$ 75.02	$ 67.05

Interest rate used to account for mortgage loan at fair value upon assumption.

No definition available.

Name:	key_InterestRateUsedToAccountForMortgageLoanAtFairValueUponAssumption
Namespace Prefix:	key_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Servicing Assets And Servicing Liabilities At Fair Value, Assumptions Used To Estimate Fair Value, Escrow Earn Rate

+ References

No definition available.

+ Details

Name:	key_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueEscrowEarnRate
Namespace Prefix:	key_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Servicing Assets And Servicing Liabilities At Fair Value, Assumptions Used To Estimate Fair Value, Servicing Cost

+ References

No definition available.

+ Details

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

- Definition

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481229/860-50-50-2

+ Details

Name:	us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of contractually collectible principal associated with a financial asset expected not to be collected due to default by the borrower.

+ References

+ Details

Name:	us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueExpectedCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated rate of prepayments of principal on servicing assets and servicing liabilities.

+ References

+ Details

Name:	us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481229/860-50-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481229/860-50-50-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481229/860-50-50-3

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(4)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481229/860-50-50-3

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481229/860-50-50-4

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
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_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: