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Other Assets (Schedule of Other Long-Term Assets) (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Long-term assets:    
Advance coal royalties $ 9,629 $ 9,296
Marketable equity securities available for sale, at fair value (restricted) [1] 2,005 2,036
Other 2,637 2,782
Long-term assets $ 14,271 $ 14,114
[1] Held by Sunrise Indemnity, Inc., our wholly owned captive insurance company.