Unconsolidated Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	$ 28,978	$ 24,611
Statement of Operations
Revenues	128,432	140,990
Gross profit	16,969	31,128
Net income	14,793	39,632
Balance Sheet
Current assets	188,146	200,668
Current liabilities	52,257	61,966
Unconsolidated mines | NACoal
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	29,000	24,600
Maximum risk of loss	6,500	5,000
Balance Sheet
Dividends from unconsolidated mines	49,700	53,500
Unconsolidated mines | NACoal | Variable Interest Entity, Not Primary Beneficiary
Statement of Operations
Revenues	768,660	734,515
Gross profit	69,021	72,433
Income before income taxes	60,398	65,183
Net income	50,933	54,067
Balance Sheet
Current assets	186,934	183,848
Assets, Noncurrent	959,032	837,477
Current liabilities	143,843	141,132
Non-current liabilities	$ 995,658	$ 875,216