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Asset Retirement Obligations - Changes to Asset Retirement Obligation (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Asset retirement obligation at beginning of period $ 27.8 $ 30.0
Accretion expense 1.3 1.5
Payments (0.8) (3.5)
Revisions in estimated cash flows [1] 3.5 (0.2)
Asset retirement obligation at end of period $ 31.8 $ 27.8
[1] For 2020, primarily consists of an adjustment of $4.5 million related to the San Lorenzo, California facility closure