Concentration of Credit Risk and Major Customers (Schedule of Concentration of Credit Risk) (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Total Trade Receivables	$ 133,788,000	$ 87,589,000
Less: Allowance for doubtful accounts	2,100,000	0
Total Trade Receivables, net of Allowance	131,688,000	87,589,000
Thermal coal utilities | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Trade Receivables	58,557,000	61,218,000
Coal exporters and distributors | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Trade Receivables	73,416,000	22,972,000
Steel and coke producers | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Trade Receivables	0	661,000
Other | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Trade Receivables	$ 1,815,000	$ 2,738,000