LONG-TERM DEBT - Schedule of Long-term Debt (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Nov. 30, 2017
Debt Instrument [Line Items]
Long-term debt	$ 851,592,000	$ 875,968,000
Less: Unamortized Debt Issuance Costs	19,905,000	21,129,000
Less amounts due in one year	8,069,000	19,318,000
Long-Term Debt	843,523,000	856,650,000
Current portion of capital lease obligations	10,317,000	3,164,000
Loans Payable \| Term Loan B
Debt Instrument [Line Items]
Long-term debt	391,547,000	392,147,000
Principal	399,000,000	400,000,000
Unamortized discount	$ 7,453,000	7,853,000
Weighted average interest rate (as a percent)	7.99%
Loans Payable \| MEDCO Revenue Bonds in Series due September 2025 at 5.75%
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 102,865,000	102,865,000
Stated interest rate (as a percent)	5.75%
Loans Payable \| Term Loan A
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 85,000,000	100,000,000
Principal	$ 100,000,000
Weighted average interest rate (as a percent)	6.41%
Loans Payable \| Advance royalty commitments
Debt Instrument [Line Items]
Long term debt, carrying amount	$ 2,085,000	2,085,000
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	$ 290,000,000	$ 300,000,000
Principal	$ 300,000,000		$ 300,000,000
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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Face (par) amount of debt instrument at time of issuance.

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

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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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