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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 Analysis [Roll Forward]    
Net asset retirement obligation at January 1 $ 28.1 $ 28.1
Liabilities acquired 1.7 0.0
Liabilities incurred 3.4 1.2
Liabilities settled (0.1) (2.8)
Accretion expense (1.7) (1.6)
Net asset retirement obligation at December 31 34.8 28.1
Current ARO liabilities $ 1.5 $ 0.5