Unconsolidated Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	$ 20,091	$ 16,335
Unconsolidated mines | NACoal
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	20,100	16,300
Maximum risk of loss	4,400	5,200
Statement of Operations
Revenue	766,558	791,264
Gross profit	76,600	87,760
Income before income taxes	66,270	62,607
Net income	55,247	55,268
Balance Sheet
Current assets	182,353	179,316
Non-current assets	860,049	883,919
Current liabilities	146,788	175,844
Non-current liabilities	891,175	882,200
Dividends from unconsolidated mines	$ 56,000	$ 54,700