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Derivatives and Fair Value Measurements (Details 4) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Level 1 to Level 2 transfers $ 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount $ 0us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount  
Level 2 to Level 1 transfers 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount 0us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount  
Carrying value      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt, carrying value 6,391.6us-gaap_DebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
  5,986.8us-gaap_DebtAndCapitalLeaseObligations
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Estimated fair value      
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]      
Long-term debt, fair value $ 4,706.3us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
  $ 5,227.9us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember