Risk Concentrations (Narrative) (Details) T in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) T	Dec. 31, 2015 USD ($)
Concentration Risk [Line Items]
Tons of coal sold | T	93.9
Percentage of tons sold under long term contract	76.00%
Long term contracts percentage of company revenue	62.00%
Long-term contracts, life term, minimum	1 year
Long-term contracts, life term, maximum	5 years
Electric Utilities [Member]
Concentration Risk [Line Items]
Receivables	$ 96.0	$ 83.8
Percentage of total trade accounts receivable	52.00%	72.00%
Domestic And Foreign Steel Producers [Member]
Concentration Risk [Line Items]
Trade accounts receivable (net of allowance for doubtful accounts of $0.0 million and $7.8 million, respectively)	$ 88.0	$ 32.8
Trade receivables, percentage	48.00%	28.00%