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Self Insurance	12 Months Ended
Self Insurance	Dec. 31, 2017
Self Insurance [Abstract]
Self Insurance	(8) Self Insurance We self-insure our underground mining equipment. Such equipment is allocated among 10 mining units spread out over 22 miles. The historical cost of such equipment is about $258 million. As of December 31, 2017, and 2016, restricted cash of $3.8 million and $2.8 million, respectively, represents cash held and controlled by a third party and is restricted for future workers’ compensation claim payments.