Quarterly report pursuant to Section 13 or 15(d)

FAIR VALUE - Fair Value of Other Financial Instruments (Details)

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FAIR VALUE - Fair Value of Other Financial Instruments (Details) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 980,800,000 $ 979,324,222
Asset retirement obligations 200,000  
Estimated Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair value of senior unsecured notes 642,000,000  
8% Senior Notes | Senior Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 692,800,000 $ 691,954,489