Sales Contracts (Schedule of Sales Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Acquired fair value, Assets	$ 97,196	$ 97,196
Acquired fair value, Liabilities	31,742	31,742
Accumulated amortization, Asset	(84,760)	(25,625)
Accumulated amortization, Liabilities	(29,979)	(24,829)
Total, Assets	12,436	71,571
Total, Liabilities	1,763	6,913
Net total, Liabilities	10,673	64,658
Coal Supply Agreement, Liabilities	934	5,114
Other current assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	12,432	59,702
Other current liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	934	5,114
Other noncurrent assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	4	11,869
Other noncurrent liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	$ 829	$ 1,799