Concentration of Credit Risk and Major Customers (Schedule of Concentration of Credit Risk) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Total Accounts Receivable Trade	$ 131,545	$ 95,707
Accounts Receivable [Member] | Customer Concentration Risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	131,545	95,707
Thermal coal utilities | Accounts Receivable [Member] | Customer Concentration Risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	69,550	62,525
Coal brokers and distributors | Accounts Receivable [Member] | Customer Concentration Risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	56,146	28,955
Other | Accounts Receivable [Member] | Customer Concentration Risk
Concentration Risk [Line Items]
Total Accounts Receivable Trade	$ 5,849	$ 4,227