Property, Plant, Equipment and Mine Development - (Details) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Property, Plant, Equipment and Mine Development[Line Items]
Less: Accumulated depreciation, depletion and amortization	$ (1,480,500,000)	$ (1,321,300,000)
Property, plant, equipment and mine development, net	2,950,600,000	3,051,100,000
Coal reserves	1,400,000,000	1,500,000,000
Coal reserves not subject to depletion	100,000,000	100,000,000
Miscellaneous Receivables
Property, Plant, Equipment and Mine Development[Line Items]
Accounts receivable, allowance for credit losses	800,000,000	800,000,000
Land and coal interests
Property, Plant, Equipment and Mine Development[Line Items]
Property, plant equipment and mine development, gross	2,494,100,000	2,482,900,000
Buildings and improvements
Property, Plant, Equipment and Mine Development[Line Items]
Property, plant equipment and mine development, gross	550,800,000	481,000,000.0
Machinery and equipment
Property, Plant, Equipment and Mine Development[Line Items]
Property, plant equipment and mine development, gross	1,386,500,000	1,408,500,000
Coal Reserves Held by Fee Ownership
Property, Plant, Equipment and Mine Development[Line Items]
Coal reserves	$ 600,000,000	$ 700,000,000