Concentration of Credit Risk and Major Customers (Schedule of Concentration of Credit Risk) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Total Accounts Receivable Trade	$ 87,589	$ 131,545
Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	87,589	131,545
Thermal coal utilities | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	61,218	69,550
Coal exporters and distributors | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	22,972	56,146
Steel and coke producers | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	661	0
Other | Accounts receivable | Customer concentration risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	$ 2,738	$ 5,849