Sales Contracts (Schedule of Sales Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
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	(88,130)	(84,760)
Accumulated amortization, Liabilities	(30,298)	(29,979)
Total, Assets	9,066	12,436
Total, Liabilities	1,444	1,763
Net total, Liabilities	7,622	10,673
Coal Supply Agreement, Liabilities	780	934
Other current assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	9,062	12,432
Other Current Liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	780	934
Other Noncurrent Assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	4	4
Other Noncurrent Liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	$ 664	$ 829