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ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Recognized in Balance Sheet (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
WORKERS' COMPENSATION AND PNEUMOCONIOSIS      
Black lung claims $ 64,988 $ 61,693  
Workers' compensation claims 48,131 54,558 $ 57,557
Total obligations 113,119 116,251  
Less current portion (9,897) (8,688)  
Non-current obligations $ 103,222 $ 107,563