Financing Receivable, Allowance For Credit Loss, Percentage

No definition available.

Name:	key_FinancingReceivableAllowanceForCreditLossPercentage
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing Receivable, Modifications, CARES Act

+ References

No definition available.

+ Details

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

- Definition

Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Period Past Due

+ References

No definition available.

+ Details

Name:	key_FinancingReceivableTroubledDebtRestructuringSubsequentDefaultPeriodPastDue
Namespace Prefix:	key_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Net reduction to interest income.

+ References

No definition available.

+ Details

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

- Definition

Percentage of carrying amount of our commercial nonperforming loans outstanding.

+ References

No definition available.

+ Details

Name:	key_PercentageOfCarryingAmountOfOurCommercialNonperformingLoansOutstanding
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of loans held for sale and other nonperforming assets.

+ References

No definition available.

+ Details

Name:	key_PercentageOfLoansHeldForSaleAndOtherNonperformingAssets
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of nonperforming loans outstanding face value.

+ References

No definition available.

+ Details

Name:	key_PercentageOfNonperformingLoansOutstandingFaceValue
Namespace Prefix:	key_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Significant commitments to lend additional funds to borrowers with loans on nonperforming status.

+ References

No definition available.

+ Details

Name:	key_SignificantCommitmentsToLendAdditionalFundsToBorrowersWithLoansOnNonperformingStatus
Namespace Prefix:	key_
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before allowance for credit loss, of accrued interest on financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL120267963-210447

+ Details

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

- Definition

Amount of increase (decrease) in allowance for credit loss on financing receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of financing receivable on nonaccrual status with no allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919258-210447

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919230-210447

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922355-210448

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
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_StatementOperatingActivitiesSegmentAxis=us-gaap_SegmentContinuingOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Asset Quality - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) SecurityLoan	Sep. 30, 2019 USD ($) SecurityLoan	Sep. 30, 2020 USD ($) SecurityLoan	Sep. 30, 2019 USD ($) SecurityLoan	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income (residential)	$ 103,081		$ 103,081		$ 94,646
Loan modifications and forbearance, CARES Act			4,500
Accrued interest	$ 235		$ 235		244
Percentage of carrying amount of our commercial nonperforming loans outstanding	77.00%		77.00%
Percentage of nonperforming loans outstanding face value	80.00%		80.00%
Percentage of loans held for sale and other nonperforming assets	87.00%		87.00%
Net reduction to interest income	$ 7	$ 7	$ 20	$ 24
Commitments outstanding to lend additional funds to borrowers (less than $1 million)	1		1		5
Financial receivable, modifications, subsequent default, recorded investment	1	$ 1	4	$ 5
Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income (residential)	74,481		74,481		68,032
Accrued interest	$ 133		$ 133
TDRs, period for default			90 days
Number of loans \| SecurityLoan	0	0
Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income (residential)	$ 28,600		$ 28,600		26,614
Accrued interest	$ 102		$ 102
TDRs, period for default			60 days
Number of loans \| SecurityLoan	33	79
Continuing operations \| Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in ALLL	$ 68
Increase in ALLL, percent	6.10%
Continuing operations \| Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in ALLL	$ (46)
Increase in ALLL, percent	(7.70%)
PPP
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income (residential)	$ 8,000		$ 8,000
Real estate — residential mortgage \| Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned income (residential)	8,715		$ 8,715		7,023
Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans \| SecurityLoan			160	255
Commercial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans \| SecurityLoan			0	0
Nonperforming financial instruments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonperforming loans on nonaccrual status with no allowance	$ 456		$ 456
Real estate — residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financial receivable, modifications, subsequent default, recorded investment			$ 99		$ 97