Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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	$ 0.3
Charged to Costs and Expenses	0.0	0.0	0.0
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	10.4	7.9	0.2
Charged to Costs and Expenses	0.6	3.5	8.9
Deductions	(2.0)	(1.0)	(1.2)
Other	0.0	0.0	0.0
Balance at End of Period	9.0	10.4	7.9
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
Charged to Costs and Expenses			(4.4)
Deductions			0.0
Other			0.0
Balance at End of Period			0.0
Tax valuation allowances
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	2,287.3	2,068.4	2,094.3
Charged to Costs and Expenses	(121.7)	373.2	(29.8)
Deductions	0.0	0.0	0.0
Other	(44.8)	(154.3)	3.9
Balance at End of Period	$ 2,120.8	$ 2,287.3	$ 2,068.4