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Commitments and Contingencies, Rollforward of CARO Liabilities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]      
Weighted Average Discount Rate, Percent 10.00% 10.00%  
Asset Retirement Obligation, Roll Forward      
Beginning balance $ 5.5 $ 4.9 $ 4.8
Liabilities incurred during the period 0.0 0.0 0.0
Liabilities settled during the period 0.0 (0.1) (0.2)
Accretion expense 0.4 0.5 0.3
Adjustment to accretion expense due to revisions to estimated cash flow and timing of expenditure1 0.0 0.2 0.0
Ending balance $ 5.9 $ 5.5 $ 4.9