Coal Mining Business - Assets and Liabilities Held for Sale (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Disposal Group, Including Discontinued Operation, Assets, Current [Abstract]
Total current assets held for sale	$ 0.8	$ 0.9
Total assets held for sale	0.8		$ 12.3
Disposal Group, Including Discontinued Operation, Liabilities, Current [Abstract]
Total current liabilities held for sale	6.7	0.9
Coal Mining | Coal Mining Business | Disposal Group, Held-for-sale, Not Discontinued Operation
Disposal Group, Including Discontinued Operation, Assets, Current [Abstract]
Receivables	0.6	0.6
Inventories	0.2	0.3
Properties, plants and equipment, net	10.7	0.0
Valuation allowance	(10.7)	0.0
Total current assets held for sale	0.8	0.9
Properties, plants and equipment, net	0.0	11.4
Total assets held for sale	0.8	12.3
Disposal Group, Including Discontinued Operation, Liabilities, Current [Abstract]
Accounts payable	0.1	0.1
Accrued liabilities	0.7	0.8
Asset retirement obligations	5.9	0.0
Total current liabilities held for sale	6.7	0.9
Asset retirement obligations	0.0	5.9
Total liabilities held for sale	$ 6.7	$ 6.8