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LONG-TERM DEBT - 2025 Senior Notes (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Apr. 24, 2017
May 31, 2017
Jun. 30, 2017
Sep. 30, 2017
Issuance of Senior Notes        
Make-whole payment       $ 8,148,000
Discount and debt issuance costs incurred       $ 16,221,000
2025 Senior Notes | ARLP Senior Notes        
Issuance of Senior Notes        
Discount and debt issuance costs incurred     $ 7,300,000  
ARLP Series B Senior Notes | ARLP Senior Notes        
Issuance of Senior Notes        
Make-whole payment   $ 8,100,000    
Intermediate Partnership and Alliance Finance | 2025 Senior Notes | ARLP Senior Notes        
Issuance of Senior Notes        
Aggregate principal amount $ 400,000,000      
Initial term 8 years      
Interest rate (as a percent) 7.50%      
Percentage of debt that may be redeemed prior to May 1, 2020 35.00%      
Redemption price expressed as a percentage of principal 107.50%