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Asset Quality - Credit Risk Profile by Regulatory Classifications (Detail) (Residential - Prime, Financing Receivable [Member], USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Mar. 31, 2014
Financing Receivable, Recorded Investment [Line Items]    
Total $ 12,742key_FinancingReceivableModificationsConcessions $ 12,764key_FinancingReceivableModificationsConcessions
Pass [Member]
    
Financing Receivable, Recorded Investment [Line Items]    
Total 12,463us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember		 12,445us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
Substandard [Member]
    
Financing Receivable, Recorded Investment [Line Items]    
Total $ 279us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember		 $ 319us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPrimeFinancingReceivableMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember