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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation [Roll Forward]    
Beginning balance, ARO liability $ 18.2 $ 28.5
Additional ARO liability incurred 2.9 3.6
ARO liabilities settled (3.5) (6.5)
Accretion expense 1.3 1.9
Revisions in estimated cash flows 0.0 (9.3)
Ending balance, ARO liability $ 18.9 $ 18.2