Subsequent Events - Sale of Coal Mining Business (Details) T in Millions, $ in Millions		3 Months Ended
	Apr. 06, 2016 USD ($) T	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)
Remaining Coal Mining Assets, Mineral Leases, Real Estate, and a Substantial Portion of Mining Reclamation Obligations | Disposal Group, Held-for-sale, Not Discontinued Operation | Coal Mining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment charge on write-down			$ 10.7
Gain (loss) on disposal of business			$ (9.2)
Subsequent Event | Coal
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Supply agreement, tons of coal | T	0.3
Supply agreement, term	5 years
Subsequent Event | Forecast | Letter of Credit
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Letters of credit relieved		$ 10.0
Subsequent Event | Remaining Coal Mining Assets, Mineral Leases, Real Estate, and a Substantial Portion of Mining Reclamation Obligations | Disposal Group, Held-for-sale, Not Discontinued Operation | Coal Mining
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Payments for assumption of business	$ 10.3
Subsequent Event | Remaining Coal Mining Assets, Mineral Leases, Real Estate, and a Substantial Portion of Mining Reclamation Obligations | Disposal Group, Held-for-sale, Not Discontinued Operation | Coal Mining | Forecast
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on disposal of business		(7.0)
Anticipated transaction-related costs		$ 2.0