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Asset Retirement Obligations (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Changes in AROs    
Obligation at beginning of the period $ 131.6  
Accretion expense 2.4  
Liabilities incurred 0.4  
Expenditures (2.0)  
Obligation at end of the period 132.4  
Expected deposits to be made into the trust 50.0  
Expected deposits to be made into the escrow account for the benefit of the FDEP 2.0  
Total balance in ARO trust 147.6 145.4
Current portion 16.2 13.8
Noncurrent portion 116.2 117.8
Total Asset Retirement Obligation $ 132.4