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Fair Value Measurements (Narrative) (Details) - USD ($)
3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]        
Impairments   $ 2,830,000,000 $ 16,000,000 $ 171,000,000
Montage Resources Corporation        
Debt Instrument [Line Items]        
Impairments   0    
Not Designated as Hedging Instrument        
Debt Instrument [Line Items]        
Impact of non-performance risk on fair value of the net derivative liability position   1,000,000    
Nonrecurring        
Debt Instrument [Line Items]        
Impairments       161,000,000
Nonrecurring | Marketing        
Debt Instrument [Line Items]        
Impairments       145,000,000
Nonrecurring | Exploration and Production        
Debt Instrument [Line Items]        
Impairments       15,000,000
Other non-core assets        
Debt Instrument [Line Items]        
Impairments $ 1,000,000 5,000,000 16,000,000 11,000,000
Other non-core assets | Nonrecurring        
Debt Instrument [Line Items]        
Carrying value of non core assets   6,000,000 26,000,000  
Impairments   $ 5,000,000 $ 16,000,000 $ 1,000,000
Senior Notes | 4.10% Senior Notes Due January March 2022        
Debt Instrument [Line Items]        
Stated interest rate   4.10% 4.10%