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Commitments, Contingent Liabilities and Guarantees - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Guarantee
Dec. 31, 2013
Dec. 31, 2012
Commitments Contingencies And Guarantees [Line Items]      
Rental expense under all operating leases $ 122,000,000key_RentalExpenseUnderAllOperatingLeases $ 122,000,000key_RentalExpenseUnderAllOperatingLeases $ 121,000,000key_RentalExpenseUnderAllOperatingLeases
Minimum future rental payments under noncancelable operating leases due in 2015 116,000,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent    
Minimum future rental payments under noncancelable operating leases due in 2016 102,000,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears    
Minimum future rental payments under noncancelable operating leases due in 2017 95,000,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears    
Minimum future rental payments under noncancelable operating leases due in 2018 79,000,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears    
Minimum future rental payments under noncancelable operating leases due in 2019 64,000,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears    
Minimum future rental payments under noncancelable operating leases, all subsequent years 370,000,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter    
Weighted-average remaining term for outstanding commercial mortgage loans in years 7 years 6 months    
Unpaid principal balance outstanding of loans sold 4,700,000,000us-gaap_RecordedUnconditionalPurchaseObligation    
Potential undiscounted future payments maximum limit One-third of the principal balance of loans outstanding    
Obligations under guarantees 222,000,000us-gaap_GuaranteeObligationsCurrentCarryingValue    
Number of default guarantees 0key_DefaultsUnderLoanGuarantees    
Written Put Options [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Obligations under guarantees 149,000,000us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_OpenOptionContractsWrittenTypeAxis
= us-gaap_PutOptionMember
   
Weighted average life of written put options 2 years 1 month 6 days    
Return Guarantee Agreement with LIHTC Investors [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Compliance period of property 15 years    
Obligations under guarantees 4,000,000us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= key_ReturnGuaranteeAgreementMember
   
Expiration of guarantees Extend through 2018    
Standby Letters of Credit [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Remaining weighted-average life of standby letters of credit in years 3 years 1 month 6 days    
Remaining actual life letters of credit in years From less than one year to as many as 12 years    
Obligations under guarantees $ 65,000,000us-gaap_GuaranteeObligationsCurrentCarryingValue
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= us-gaap_FinancialStandbyLetterOfCreditMember
   
Minimum [Member] | Low [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Guarantee Obligations 0.00%us-gaap_GuaranteeObligationsLiquidationProceedsPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_RiskLevelLowMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Minimum [Member] | Moderate [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Guarantee Obligations 31.00%us-gaap_GuaranteeObligationsLiquidationProceedsPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_RiskLevelMediumMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Minimum [Member] | High [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Guarantee Obligations 71.00%us-gaap_GuaranteeObligationsLiquidationProceedsPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_RiskLevelHighMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Maximum [Member] | Low [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Guarantee Obligations 30.00%us-gaap_GuaranteeObligationsLiquidationProceedsPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_RiskLevelLowMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Maximum [Member] | Moderate [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Guarantee Obligations 70.00%us-gaap_GuaranteeObligationsLiquidationProceedsPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_RiskLevelMediumMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Maximum [Member] | High [Member]      
Commitments Contingencies And Guarantees [Line Items]      
Guarantee Obligations 100.00%us-gaap_GuaranteeObligationsLiquidationProceedsPercentage
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_RiskLevelHighMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember