Risk Concentrations (Narrative) (Details) $ in Thousands, T in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) T	Dec. 31, 2016 USD ($)
Concentration Risk [Line Items]
Trade accounts receivable	$ 172,604	$ 184,483
Tons of coal sold | T	98.2
Percentage of tons sold under long term contract	66.00%
Long term contracts percentage of company revenue	55.00%
Long-term contracts, life term, minimum	1 year
Long-term contracts, life term, maximum	4 years
Electric utilities
Concentration Risk [Line Items]
Receivables	$ 72,900	$ 96,000
Percentage of total trade accounts receivable	42.00%	52.00%
Domestic and foreign steel producers
Concentration Risk [Line Items]
Trade accounts receivable	$ 99,400	$ 88,000
Trade receivables, percentage	58.00%	48.00%