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INVESTMENTS - AllDale (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 28, 2017
Nov. 30, 2014
Changes in equity method investment          
Beginning balance $ 138,817        
Equity investment income (loss) 13,860 $ 3,543 $ (49,046)    
Ending balance 147,964 138,817      
AllDale Minerals | ARLP          
Changes in equity method investment          
Beginning balance 138,817 64,509 11,257    
Contributions 20,688 76,797 54,290    
Equity investment income (loss) 13,860 3,543 (594)    
Distributions received (25,401) (6,032) (444)    
Ending balance $ 147,964 $ 138,817 $ 64,509    
Funding Commitments | All Dale Minerals III | Alliance Minerals          
Equity method investments, additional information          
Investment commitment       $ 30,000 $ 30,000