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Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Changes in AROs    
Obligation at beginning of the period $ 145.0  
Accretion expense 7.8  
Liabilities incurred 1.1  
Expenditures (3.6)  
Changes in estimates 7.8  
Obligation at end of the period 158.1  
Change of estimate recorded as an increase to property, plant and equipment 7.5  
Change of estimate charged to cost of sales 0.3  
Current portion 9.4 12.3
Noncurrent portion 148.7 132.7
Total Asset Retirement Obligation 158.1  
Expected deposits to be made into the escrow account for the benefit of the FDEP 4.0  
Total balance in ARO trust $ 200.8 $ 200.8