Risk Concentrations (Narrative) (Details) $ in Thousands, T in Millions	12 Months Ended
Dec. 31, 2021 USD ($) T	Dec. 31, 2020 USD ($)
Concentration Risk [Line Items]
Trade accounts receivable	$ 324,304	$ 110,869
Tons of coal sold \| T	73.0
Percentage of tons sold under long term contract	63.00%
Long term contracts percentage of company revenue	35.00%
Long-term contracts, life term, minimum	1 year
Long-term contracts, life term, maximum	5 years
Electric utilities
Concentration Risk [Line Items]
Receivables	$ 72,800	$ 41,700
Percentage of total trade accounts receivable	22.00%	38.00%
Domestic and foreign steel producers
Concentration Risk [Line Items]
Trade accounts receivable	$ 251,500	$ 69,100
Trade receivables, percentage	78.00%	62.00%

The total amount due to the entity within one year of the balance sheet date (or one operating cycle, if longer) from outside sources, including trade accounts receivable, notes and loans receivable, as well as any other types of receivables, net of allowances established for the purpose of reducing such receivables to an amount that approximates their net realizable value.

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Balance Type:	debit
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