Income Taxes Components of Net Deferred Tax Liability (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Retirement benefit liabilities	$ 13.4	$ 14.6
Black lung benefit liabilities	19.4	16.8
Share-based compensation	8.4	6.9
Federal tax credit carryforward	23.0	19.8
Foreign tax credit carryforward	8.9	0.0
Federal net operating loss	8.2	0.0
State tax credit carryforward, net of federal income tax effects	6.4	9.2
State net operating loss carryforward, net of federal income tax effects	7.4	5.4
Other liabilities not yet deductible	12.0	19.4
Properties, plants and equipment	12.0	0.0
Investment in subsidiaries	0.0	11.8
Total deferred tax assets	119.1	103.9
Less valuation allowance	(5.8)	(14.7)
Deferred tax asset, net	113.3	89.2
Deferred tax liabilities:
Properties, plants and equipment	0.0	(85.0)
Investment in partnerships	(462.3)	(299.7)
Total deferred tax liabilities	(462.3)	(384.7)
Net deferred tax liability	$ (349.0)	$ (295.5)