FAIR VALUE OF FINANCIAL INSTRUMENTS - Schedule of Financial Instruments Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Murray Energy Guarantees	$ (818)	$ (1,040)
Fair Value Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Murray Energy Guarantees	0	0
Fair Value Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Murray Energy Guarantees	0	0
Fair Value Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Murray Energy Guarantees	$ (818)	$ (1,040)