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WORKERS' COMPENSATION AND PNEUMOCONIOSIS - Workers' Compensation Liability (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2017
Sep. 30, 2017
Sep. 30, 2016
Jun. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of the changes in workers' compensation liability                
Beginning balance   $ 59,981 $ 56,403 $ 54,558 $ 48,131 $ 54,558 $ 54,558  
Accruals increase   1,999 2,480   16,673 7,904    
Payments   (2,357) (2,238)   (7,799) (8,226)    
Interest accretion   421 492   1,261 1,476    
Valuation loss         1,778 1,425    
Ending balance $ 59,981 $ 60,044 $ 57,137 $ 56,403 $ 60,044 $ 57,137 $ 48,131 $ 54,558
Workers' compensation discount rate 3.38%     2.89%     3.52% 3.63%