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Asset Retirement Obligations Asset Retirement Obligations ARO Rollforward (Details)
$ in Millions
6 Months Ended
Jun. 30, 2020
USD ($)
Asset Retirement Obligation Disclosure [Abstract]  
ARO liability at January 1, 2020 $ 100.9
Accretion 1.9
Additions 1.0
Revisions (0.5)
Liabilities settled (0.9)
Liabilities related to assets sold (1.3)
ARO Liability at June 30, 2020 $ 101.1