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Income Taxes (Components of Net Deferred Tax Liability) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Deferred tax assets:    
Retirement benefit liabilities $ 6.3 $ 6.4
Black lung benefit liabilities 14.8 12.8
Share-based compensation 4.2 4.7
Federal tax credit carryforward 19.9 20.5
Foreign tax credit carryforward 19.4 14.4
Federal net operating loss 5.3 1.6
Section 163(j) interest limitation carryforward 0.0 5.7
State tax credit carryforward, net of federal income tax effects 0.3 1.1
State net operating loss carryforward, net of federal income tax effects 12.9 13.6
Other liabilities not yet deductible 11.5 4.4
Total deferred tax assets 94.6 85.2
Less valuation allowance (19.6) (20.9)
Deferred tax asset, net 75.0 64.3
Deferred tax liabilities:    
Properties, plants and equipment (152.9) (17.9)
Investment in partnerships (81.4) (194.0)
Total deferred tax liabilities (234.3) (211.9)
Net deferred tax liability (159.3) $ (147.6)
Provisional income tax expense (benefit) $ 10.7