Unconsolidated Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	$ 16,335	$ 31,054
Unconsolidated mines | NACoal
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	16,300	31,100
Maximum risk of loss	5,200	4,600	$ 4,000
Statement of Operations
Revenues	791,264	649,050	608,349
Gross profit	87,760	80,068	71,727
Income before income taxes	62,607	54,857	49,641
Net income	55,268	40,590	$ 39,181
Balance Sheet
Current assets	179,316	160,554
Non-current assets	883,919	901,221
Current liabilities	175,844	127,361
Non-current liabilities	882,200	929,774
Dividends from unconsolidated mines	$ 54,700	$ 39,900