LONG-TERM DEBT - Schedule of Long-term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Nov. 30, 2017
Debt Instrument [Line Items]
Long-term debt	$ 830,801	$ 875,968
Less: Unamortized Debt Issuance Costs	18,322	21,129
Less amounts due in one year	4,318	19,318
Long-Term Debt	826,483	856,650
Current portion of capital lease obligations	16,440	3,164
Loans Payable \| Term Loan B
Debt Instrument [Line Items]
Long-term debt	390,947	392,147
Long term debt, carrying amount	398,000	400,000
Unamortized discount	$ 7,053	7,853
Weighted average interest rate (as a percent)	8.32%
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	$ 73,750	100,000
Weighted average interest rate (as a percent)	6.67%
Loans Payable \| Advance royalty commitments
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 2,085	2,085
Weighted average interest rate (as a percent)	9.42%
Senior Notes \| Senior Secured Second Lien Notes due 2025
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 279,476	$ 300,000
Stated interest rate (as a percent)	11.00%		11.00%

Amount of capital lease obligation due within one year or the normal operating cycle, if longer.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Weighted average interest rate of debt outstanding.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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