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Fair Market Value of Total Debt (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Debt Instrument [Line Items]    
Fair market value of total debt $ 5,154us-gaap_DebtInstrumentFairValue $ 4,933us-gaap_DebtInstrumentFairValue
Level 1    
Debt Instrument [Line Items]    
Fair market value of total debt 1,413us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 1,589us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Level 2    
Debt Instrument [Line Items]    
Fair market value of total debt $ 3,741us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 $ 3,344us-gaap_DebtInstrumentFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member