Unconsolidated Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	$ 31,054	$ 24,643
Unconsolidated mines | NACoal
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	31,100	24,600
Maximum loss	4,600	4,000	$ 4,000
Statement of Operations
Revenues	649,050	608,349	579,031
Gross profit	80,068	71,727	74,244
Income before income taxes	54,857	49,641	48,592
Net income	40,590	39,181	$ 37,067
Balance Sheet
Current assets	160,554	160,498
Non-current assets	901,221	913,402
Current liabilities	127,361	129,126
Non-current liabilities	929,774	940,782
Dividends from unconsolidated mines	$ 39,900	$ 39,100