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Summary of Significant Accounting Policies Asset Retirement Obligation (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at January 1 $ 63.3 $ 58.6
Accretion expense 0.7 0.6
Liabilities settled 0.0 (0.3)
Balance at March 31 $ 64.0 $ 58.9