Fair Value Measurement (Details) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset impairment	$ 10,700,000	$ 0
Fair value of Company's debt	799,500,000
Long-term debt	950,700,000		$ 1,003,800,000
Coal Mining Business | Coal Mining | Disposal Group, Held-for-sale, Not Discontinued Operation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset impairment	10,700,000
Remaining Coal Mining Assets, Mineral Leases, Real Estate, and a Substantial Portion of Mining Reclamation Obligations | Coal Mining | Disposal Group, Held-for-sale, Not Discontinued Operation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets to be disposed	0
Convent Marine Terminal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business combination, contingent consideration	4,200,000		7,900,000
Cash Equivalents | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	$ 2,000,000		$ 15,400,000