Sales Contracts (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Original fair value, Assets	$ 97,196	$ 97,196
Original fair value, Liabilities	31,742	31,742
Accumulated amortization, Asset	(96,812)	(84,760)
Accumulated amortization, Liabilities	(30,924)	(29,979)
Total, Assets	384	12,436
Total, Liabilities	818	1,763
Net total, Liabilities	(434)	10,673
Coal Supply agreement, Liabilities	570	934
Other current assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	384	12,432
Other current liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply agreement, Liabilities	570	934
Other noncurrent assets
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	0	4
Other noncurrent liabilities
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply agreement, Liabilities	$ 248	$ 829