Asset Quality - Schedule of Financial Effects of Modifications to Borrowers Experiencing Financial Difficulty (Details)	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Total commercial real estate loans \| Commercial and Industrial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Interest Rate Change	(0.43%)	(17.00%)	(0.43%)	(6.29%)
Weighted-average Term Extension (in years)	4 months 28 days	5 months 1 day	9 months 29 days	1 year 1 month 20 days
Total commercial real estate loans \| Commercial mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Interest Rate Change	0.00%	0.00%	0.00%	(1.91%)
Weighted-average Term Extension (in years)	5 months 12 days	5 months 4 days	1 year 1 month 2 days	2 months 12 days
Total commercial real estate loans \| Construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Interest Rate Change	0.00%	0.00%	0.00%	0.00%
Weighted-average Term Extension (in years)	4 months 6 days	0 years	7 days	3 years 2 months 12 days
Total consumer loans \| Real estate — residential mortgage
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Interest Rate Change	(1.63%)	(1.85%)	(1.80%)	(1.73%)
Weighted-average Term Extension (in years)	7 years	2 years 5 months 19 days	5 years 10 months 13 days	5 years 9 months 7 days
Total consumer loans \| Home equity loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Interest Rate Change	(2.85%)	(4.68%)	(2.82%)	(3.75%)
Weighted-average Term Extension (in years)	7 years 8 months 8 days	5 years 7 months 6 days	5 years 11 months 12 days	6 years 5 months 8 days
Total consumer loans \| Other consumer loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Interest Rate Change	(3.59%)	(4.18%)	(3.77%)	(3.90%)
Weighted-average Term Extension (in years)	1 year 6 months 7 days	4 months 24 days	1 year 14 days	7 months 13 days
Total consumer loans \| Credit cards
Financing Receivable, Troubled Debt Restructuring [Line Items]
Weighted-average Interest Rate Change	(1.16%)	(12.13%)	(8.46%)	(15.21%)
Weighted-average Term Extension (in years)	3 months	3 months	9 months	9 months

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.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

Reference 2: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-34

Reference 3: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-33

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage of decrease in weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

+ Details

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

- Definition

Period of increase in weighted average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=key_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=key_CommercialRealEstateCommercialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=key_RealEstateResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=key_OtherConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CreditCardReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: