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LONG-TERM DEBT - 2025 Senior Notes (Details) - USD ($)
12 Months Ended
Apr. 24, 2017
Dec. 31, 2017
Dec. 31, 2016
Issuance of Senior Notes      
Make-whole payment   $ 8,148,000  
Discount and debt issuance costs incurred   $ 16,487,000 $ 101,000
ARLP Series B Senior Notes | ARLP Senior Notes      
Issuance of Senior Notes      
Make-whole payment $ 8,100,000    
Intermediate Partnership and Alliance Finance | ARLP Senior notes due 2025 | ARLP Senior Notes      
Issuance of Senior Notes      
Aggregate principal amount $ 400,000,000    
Initial term 8 years    
Interest rate (as a percent) 7.50%    
Redemption price expressed as a percentage of principal 107.50%    
Discount and debt issuance costs incurred $ 7,300,000    
Intermediate Partnership and Alliance Finance | ARLP Senior notes due 2025 | ARLP Senior Notes | Maximum      
Issuance of Senior Notes      
Percentage of debt that may be redeemed prior to May 1, 2020 35.00%