Unconsolidated Subsidiaries (Financial Information for the Unconsolidated Mines) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Equity Method Investments [Line Items]
Accounts receivable from affiliates	$ 3,085	$ 57,421
NACoal
Schedule of Equity Method Investments [Line Items]
Accounts receivable from affiliates		53,200
Unconsolidated mines | NACoal
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated mines and related tax assets	24,600	28,200
Maximum loss	4,000	4,000	$ 5,400
Dividends from unconsolidated mines	39,100	38,300
Statement of Operations
Revenues	608,349	579,031	577,436
Gross profit	71,727	74,244	74,870
Income before income taxes	49,641	48,592	47,953
Net income	39,181	37,067	$ 37,468
Balance Sheet
Current assets	160,498	143,105
Non-current assets	913,402	781,475
Current liabilities	129,126	177,659
Non-current liabilities	$ 940,782	$ 742,938