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Other Assets (Schedule of Other Assets) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Long-term assets:    
Advance coal royalties $ 9,903 $ 10,186
Marketable equity securities available for sale at fair value (restricted)* 2,113 1,909
Other 2,107 2,122
Total other assets $ 14,123 $ 14,217