Sales Contracts (Schedule of Sales Contracts) (Details) - USD ($) $ in Thousands	Mar. 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	(44,843)	(25,625)
Accumulated amortization, Liabilities	(29,356)	(24,829)
Total, Assets	52,353	71,571
Total, Liabilities	2,386	6,913
Net total, Liabilities	49,967	64,658
Coal Supply Agreement, Liabilities	817	5,114
Other Current Assets [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	43,238	59,702
Other Current Liabilities [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	817	5,114
Other Noncurrent Assets [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal supply agreement, Assets	9,115	11,869
Other Noncurrent Liabilities [Member]
Acquired Finite-Lived Intangible Assets And Liabilities By Major Class [Line Items]
Coal Supply Agreement, Liabilities	$ 1,569	$ 1,799