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ACQUISITION (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended
Nov. 13, 2020
Aug. 31, 2020
Mar. 31, 2021
Dec. 31, 2020
Montage Resources Corporation        
Business Acquisition [Line Items]        
Number of Southwestern Energy common stock for each share of Montage Resources Corporation common stock converted (in shares) 1.8656      
Offering price (in dollars per share) $ 3.05      
Percentage of voting interest 90.00%      
Revenues associated with the operations acquired through the Merger     $ 130  
Net income attributable to common stockholders associated with the operations acquired through the Merger     61  
Employee severance and related costs     $ 1  
Montage Resources Corporation | Public Stock Offering        
Business Acquisition [Line Items]        
Offering price (in dollars per share)   $ 2.50    
Underwritten public offering of common stock (in shares)   63,250,000    
Net proceeds from public offering   $ 152    
Montage Resources Corporation | Montage Resources Corporation        
Business Acquisition [Line Items]        
Percentage of voting interest 10.00%      
Senior Notes | 8.375% Senior Notes due September 2028        
Business Acquisition [Line Items]        
Senior notes   $ 350    
Stated interest rate   8.375% 8.375% 8.375%
Senior Notes | Montage Resources Corporation | Eight Point Eight Seven Five Percent Senior Notes Due 2023        
Business Acquisition [Line Items]        
Senior notes   $ 510    
Stated interest rate   8.875%    
Senior Notes | Montage Resources Corporation | 8.375% Senior Notes due September 2028        
Business Acquisition [Line Items]        
Senior notes   $ 350    
Stated interest rate   8.375%