Unconsolidated Subsidiaries (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	$ 19,090	$ 28,978
Statement of Operations
Revenues	191,846	128,432
Gross profit	43,452	16,969
Income before income taxes	56,850	14,258
Net income	48,125	14,793
Balance Sheet
Current assets	203,053	188,146
Current liabilities	46,016	52,257
Unconsolidated mines | NACoal
Schedule of Equity Method Investments [Line Items]
Investment in unconsolidated subsidiaries	19,100	29,000
Maximum risk of loss	7,600	6,500
Balance Sheet
Dividends from unconsolidated mines	51,700	49,700
Unconsolidated mines | NACoal | Variable Interest Entity, Not Primary Beneficiary
Statement of Operations
Revenues	764,759	768,660
Gross profit	68,076	69,021
Income before income taxes	60,865	60,398
Net income	53,248	50,933
Balance Sheet
Current assets	168,669	186,934
Assets, Noncurrent	900,924	959,032
Current liabilities	98,887	143,843
Non-current liabilities	$ 963,128	$ 995,658