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Loans and Loans Held for Sale - Loans Held for Sale (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Loans [Line Items]      
Commercial, financial and agricultural $ 63us-gaap_LoansHeldForSaleCommercialAndIndustrial $ 278us-gaap_LoansHeldForSaleCommercialAndIndustrial  
Commercial lease financing 15key_LoansHeldForSaleCommercialLeaseFinancing 9key_LoansHeldForSaleCommercialLeaseFinancing  
Total loans held for sale 734us-gaap_LoansReceivableHeldForSaleNet 611us-gaap_LoansReceivableHeldForSaleNet 599us-gaap_LoansReceivableHeldForSaleNet
Commercial Mortgage [Member]      
Loans [Line Items]      
Real estate - mortgage 638us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 307us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Residential Mortgage [Member]      
Loans [Line Items]      
Real estate - mortgage $ 18us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialMortgageMember		 $ 17us-gaap_LoansHeldForSaleMortgages
/ us-gaap_FinancialInstrumentAxis
= us-gaap_ResidentialMortgageMember