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LONG-TERM DEBT - Schedule of Long-term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Nov. 30, 2017
Debt Instrument [Line Items]
Long-term debt	$ 717,250	$ 826,490
Less: Unamortized Debt Issuance Costs	12,083	16,409
Less amounts due in one year	20,636	117,954
Long-Term Debt	696,614	708,536
Current portion of finance lease obligations	17,338
Current portion of capital lease obligations		16,858
Loans Payable \| Term Loan B
Debt Instrument [Line Items]
Long-term debt	273,001	389,747
Long term debt, carrying amount	274,313	396,000
Unamortized discount	$ 1,312	$ 6,253
Weighted average interest rate (as a percent)	6.91%	8.53%
Loans Payable \| MEDCO Revenue Bonds in Series due September 2025 at 5.75%
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 102,865	$ 102,865
Stated interest rate (as a percent)	5.75%
Loans Payable \| Term Loan A
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 96,250	$ 73,750
Weighted average interest rate (as a percent)	6.16%	6.78%
Loans Payable \| Advance royalty commitments
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 2,261	$ 2,261
Weighted average interest rate (as a percent)	8.57%	8.57%
Senior Notes \| Senior Secured Second Lien Notes due 2025
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 254,956	$ 274,276
Stated interest rate (as a percent)	11.00%		11.00%	11.00%