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Valuation and Qualifying Accounts (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Advance royalty recoupment reserve      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at Beginning of Period $ 0.3 $ 0.3 $ 0.0
Charged to Costs and Expenses 0.0 0.0 0.3
Deductions 0.0 0.0 0.0
Other 0.0 0.0 0.0
Balance at End of Period 0.3 0.3 0.3
Reserve for materials and supplies      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at Beginning of Period 7.9 0.2 0.6
Charged to Costs and Expenses 3.5 8.9 0.5
Deductions (1.0) (1.2) (0.9)
Other 0.0 0.0 0.0
Balance at End of Period 10.4 7.9 0.2
Allowance for credit losses      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at Beginning of Period 0.0 4.4 4.6
Charged to Costs and Expenses   (4.4) (0.2)
Deductions   0.0 0.0
Other   0.0 0.0
Balance at End of Period   0.0 4.4
Tax valuation allowances      
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance at Beginning of Period 2,068.4 2,094.3 2,432.5
Charged to Costs and Expenses 373.2 (29.8) (275.0)
Deductions 0.0 0.0 0.0
Other (154.3) 3.9 (63.2)
Balance at End of Period $ 2,287.3 $ 2,068.4 $ 2,094.3