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Fair Value Measurements - Additional Information (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loans held for sale adjusted to fair value $ 0us-gaap_LoansHeldForSaleFairValueDisclosure $ 0us-gaap_LoansHeldForSaleFairValueDisclosure $ 0us-gaap_LoansHeldForSaleFairValueDisclosure
Fair value of OREO property re-evaluated period 90 days    
Loan, net of allowance 57,159,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount 56,587,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount 54,611,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
Loans in portfolio 187,000,000key_LoanSecuritizationTrustVies    
Average maturity of short-term financial instruments Less than six months    
Residential Real Estate Mortgage Loans [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loan, net of allowance 2,200,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_MortgageLoansOnRealEstateMember		 2,200,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_MortgageLoansOnRealEstateMember		 2,200,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_MortgageLoansOnRealEstateMember
Fair Value [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loans held for sale adjusted to fair value 1,649,000,000us-gaap_LoansHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 734,000,000us-gaap_LoansHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 401,000,000us-gaap_LoansHeldForSaleFairValueDisclosure
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair Value [Member] | Discontinued Operations [Member] | Education Lending [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loan, net of allowance 2,000,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember		 1,800,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember		 1,900,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Loans in portfolio 187,000,000key_LoanSecuritizationTrustVies
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember		 191,000,000key_LoanSecuritizationTrustVies
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember		 143,000,000key_LoanSecuritizationTrustVies
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Loans in the trusts     1,900,000,000key_LoansInTrusts
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Securities issued by the education lending securitization trusts     1,800,000,000key_SecuritiesIssuedBySecuritizationTrust
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember
Carrying Amount [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loan, net of allowance 57,159,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 56,587,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 54,611,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Amount [Member] | Discontinued Operations [Member] | Education Lending [Member]      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Loan, net of allowance $ 1,700,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember		 $ 2,100,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember		 $ 2,300,000,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= key_EducationLendingMember
/ us-gaap_StatementOperatingActivitiesSegmentAxis
= us-gaap_SegmentDiscontinuedOperationsMember