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Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fair Value Disclosures [Abstract]    
Carrying value of long-term debt $ 3,488.3us-gaap_LongTermDebt $ 3,616.3us-gaap_LongTermDebt
Estimated fair value of long-term debt $ 3,640.7us-gaap_LongTermDebtFairValue $ 3,856.9us-gaap_LongTermDebtFairValue