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Asset Retirement Obligations (Details) (USD $)
In Millions
3 Months Ended9 Months Ended
Mar. 31, 2010
Sep. 30, 2011
Sep. 30, 2010
Oct. 31, 2011
Dec. 31, 2010
Changes in AROs     
Obligation at beginning of the period $ 119.8   
Accretion expense 6.5   
Liabilities incurred 2.4   
Expenditures (4.2)   
Changes in estimates 2.0   
Obligation at end of the period 126.5   
Asset Retirement Obligations     
Current portion 10.8  8.1
Noncurrent portion 115.7  111.7
Total Asset Retirement Obligation 126.5   
Annual contributions to an escrow account established for the benefit of the FDEP3.7 3.7  
Escrow account, balance 13.2   
Deposits into the trust    54.8
Transfer from escrow account to trust for asset retirement obligation    26.9
Total balance in ARO trust 81.8   
Estimated contributions to be made in 2012   $ 50.4