Income Taxes (Components of Net Deferred Tax Liability) (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets:
Retirement benefit liabilities	$ 4.2	$ 5.5
Black lung benefit liabilities	13.2	14.4
Share-based compensation	2.0	2.6
Federal tax credit carryforward	8.7	17.6
Foreign tax credit carryforward	9.2	12.3
State net operating loss carryforward, net of federal income tax effects	12.9	12.6
Other liabilities not yet deductible	12.9	10.4
Total deferred tax assets	63.1	75.4
Less valuation allowance	(9.2)	(20.2)
Deferred tax asset, net	53.9	55.2
Deferred tax liabilities:
Properties, plants and equipment	(155.4)	(151.0)
Investment in partnerships	(70.8)	(73.2)
Total deferred tax liabilities	(226.2)	(224.2)
Net deferred tax liability	$ (172.3)	$ (169.0)