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Asset Retirement Obligations - Change in Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]    
Balance at beginning of year $ 27.7us-gaap_AssetRetirementObligation $ 13.0us-gaap_AssetRetirementObligation
Accretion expense 0.9us-gaap_AssetRetirementObligationAccretionExpense 1.1us-gaap_AssetRetirementObligationAccretionExpense
Payments (2.2)us-gaap_AssetRetirementObligationLiabilitiesSettled (0.8)us-gaap_AssetRetirementObligationLiabilitiesSettled
Revision of estimates (1.0)us-gaap_AssetRetirementObligationRevisionOfEstimate 13.8us-gaap_AssetRetirementObligationRevisionOfEstimate
Liabilities incurred 0us-gaap_AssetRetirementObligationLiabilitiesIncurred 0.6us-gaap_AssetRetirementObligationLiabilitiesIncurred
Balance at end of year $ 25.4us-gaap_AssetRetirementObligation $ 27.7us-gaap_AssetRetirementObligation