ACCRUED WORKERS' COMPENSATION AND PNEUMOCONIOSIS BENEFITS - Discount Rates (Details)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accrued Workers Compensation And Pneumoconiosis Benefits
Workers' compensation discount rate	3.63%	3.41%	4.11%
Pneumoconiosis benefits
Accrued Workers Compensation And Pneumoconiosis Benefits
Pneumoconiosis discount rate	4.16%	3.82%	4.69%

Percent discount applied to worker's compensation reserve liability to reduce the reserve to present value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6482852&loc=d3e14931-158439

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.13(b))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14754-158437

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