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Loans and Loans Held for Sale - Loans Held for Sale (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Commercial, financial and agricultural $ 183us-gaap_LoansHeldForSaleCommercialAndIndustrial $ 63us-gaap_LoansHeldForSaleCommercialAndIndustrial   $ 44us-gaap_LoansHeldForSaleCommercialAndIndustrial  
Commercial lease financing 14key_LoansHeldForSaleCommercialLeaseFinancing 15key_LoansHeldForSaleCommercialLeaseFinancing   8key_LoansHeldForSaleCommercialLeaseFinancing  
Total loans held for sale 1,649us-gaap_LoansReceivableHeldForSaleNet 734us-gaap_LoansReceivableHeldForSaleNet 784us-gaap_LoansReceivableHeldForSaleNet 401us-gaap_LoansReceivableHeldForSaleNet 611us-gaap_LoansReceivableHeldForSaleNet
Commercial Mortgage [Member]          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Real estate - mortgage 1,408us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 638us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		   333us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Residential Mortgage [Member]          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Real estate - mortgage $ 44us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialMortgageMember		 $ 18us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialMortgageMember		   $ 16us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialMortgageMember