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SUPPLEMENTAL OIL AND GAS INFORMATION (UNAUDITED) (Details)
Mcf in Thousands, MBbls in Thousands, Bcfe in Thousands, Bcf in Thousands
1 Months Ended 12 Months Ended
Jul. 31, 2018
MMBTU
Feb. 28, 2018
MMBTU
Dec. 31, 2020
Bcfe
Bcf
MBbls
Mcf
Dec. 31, 2019
Bcfe
Bcf
MBbls
Dec. 31, 2018
Bcfe
Bcf
MBbls
Dec. 31, 2017
Bcfe
Bcf
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Beginning balance     12,903 11,605 9,726  
Revision of prior estimates(3)     (347) 47 780  
Extensions, discoveries and other additions(4)     1,974 2,116 2,244  
Production     (858) (865) (735)  
Sales of reserves in place (6,000) (404,000)   0 (410)  
Ending balance     13,672 12,903 11,605  
Proved Developed Reserves     8,608 8,056 7,403 6,187
Proved Undeveloped Reserves     5,064 4,847 4,202 3,539
Volume ratio | Mcf     6      
Proved Undeveloped Reserves Reclassification            
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Revision of prior estimates(3)     (66) (18) (345)  
Performance Revision            
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Revision of prior estimates(3)       67    
Performance Revisions And Drilling            
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Revision of prior estimates(3)     123 417    
Certain Proved Developed Producing Properties            
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Revision of prior estimates(3)     (368) (350)    
Pennsylvania            
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Revision of prior estimates(3)         1,123  
Extensions, discoveries and other additions(4)     1,974 2,116 2,243  
Natural Gas            
Proved Developed and Undeveloped Reserves [Roll Forward]            
Beginning balance | Bcf     12,903 11,604 9,353  
Revision of prior estimates(3) | Bcf     (347) 48 776  
Extensions, discoveries and other additions(4) | Bcf     1,974 2,116 2,243  
Production | Bcf     (858) (865) (730)  
Sales of reserves in place(5) | Bcf       0 (38)  
Ending balance | Bcf     13,672 12,903 11,604  
Proved Developed Reserves | Bcf     8,608 8,056 7,402 6,001
Proved Undeveloped Reserves | Bcf     5,064 4,847 4,202 3,352
Crude Oil and Natural Gas Liquids (NGL)            
Proved Developed and Undeveloped Reserves [Roll Forward]            
Beginning balance | MBbls     22 120 62,252  
Revision of prior estimates(3) | MBbls     (3) (48) 677  
Extensions, discoveries and other additions(4) | MBbls     0 0 0  
Production | MBbls     (4) 0 (829)  
Sales of reserves in place(5) | MBbls       (50) (61,980)  
Ending balance | MBbls     15 22 120  
Proved Developed Reserves | Bcf     15 22 107 31,066
Proved Undeveloped Reserves | Bcf     0 0 13 31,186
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Percentage of reserves           13.70%
Natural Gas Liquids            
Proved Developed and Undeveloped Reserve (Energy) [Roll Forward]            
Percentage of reserves           1.00%