Coal Mining Business (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments for assumption of business			$ 12.8	$ 0.0
Assets held for sale	$ 0.0		0.0		$ 12.3
Disposal Group, Held-for-sale, Not Discontinued Operation | Coal Mining | Coal Mining Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Assets held for sale					12.3
Properties, plants and equipment, net held for sale					11.4
Liabilities held for sale					6.8
Asset retirement obligations held for sale					$ 5.9
Disposal Group, Held-for-sale, Not Discontinued Operation | Coal Mining | Remaining Coal Mining Assets, Mineral Leases, Real Estate, and a Substantial Portion of Mining Reclamation Obligations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments for assumption of business	$ 0.7	$ 12.1
Disposal Group, Held-for-sale, Not Discontinued Operation | Coal Mining | Remaining Coal Mining Assets, Mineral Leases, Real Estate, and a Substantial Portion of Mining Reclamation Obligations | Loss on Divestiture of Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on disposal of business			(14.7)
Impairment charge on write-down			10.7
Transaction related costs			$ 1.1