REVENUE (Details) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) mines	Dec. 31, 2017 USD ($)
Disaggregation of Revenue [Line Items]
Number of mines that produce thermal coal \| mines	1
Contract assets	$ 254,506	$ 274,699
Revenue recognized, performance obligation satisfied	245,700	$ 251,200
Transfers to Receivables	32,488
Material Rights from Customers' Commitments to Pay for Final Reclamation
Disaggregation of Revenue [Line Items]
Revenue, remaining performance obligation	$ 100,100

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130533-203044

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Amount of decrease in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time from transfer to receivable due to right to consideration becoming unconditional.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of revenue recognized from performance obligation satisfied or partially satisfied in previous reporting periods. Includes, but is not limited to, change in transaction price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Number of Mines that Produce Thermal Coal

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