v3.20.2
Fair Value Measurement - Range and arithmetic average of significant unobservable inputs (Details) - Wind River Basin [Member] - Fair Value, Inputs, Level 3 [Member] - Fair Value Measurements Nonrecurring [Member] - Lower 48 [Member]
$ in Millions
 1 Months Ended 9 Months Ended
Jan. 01, 2035
Mar. 31, 2020
USD ($)
$ / MMBTU
MMcf
Sep. 30, 2020
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Fair Value Measurements, Nonrecurring, Measurement Period     During the first quarter of 2020
Fair Value Measurements, Valuation Processes, Description     The carrying value was written down to fair value. The fair value was estimated based on an internal discounted cash flow model using estimates of future production, an outlook of future prices using a combination of exchanges (short-term) and external pricing services companies (long-term), future operating costs and capital expenditures, and a discount rate believed to be consistent with those used by principal market participants.
Assets, Fair Value Disclosure | $   $ 77  
Valuation Technique, Discounted Cash Flow [Member] | Minimum [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Natural gas production | MMcf   8.4  
Valuation Technique, Discounted Cash Flow [Member] | Maximum [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Natural gas production | MMcf   55.2  
Valuation Technique, Discounted Cash Flow [Member] | Arithmetic Average [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Natural gas production | MMcf   22.9  
Measurement Input, Commodity Future Price [Member] | Forecast [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Future prices annual increase (decrease), percentage 220.00%    
Measurement Input, Commodity Future Price [Member] | Minimum [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Natural gas price outlook | $ / MMBTU   2.67  
Measurement Input, Commodity Future Price [Member] | Maximum [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Natural gas price outlook | $ / MMBTU   9.17  
Measurement Input, Commodity Future Price [Member] | Arithmetic Average [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Natural gas price outlook | $ / MMBTU   5.68  
Measurement Input, Discount Rate [Member] | Minimum [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Discount Rate   7.90%  
Measurement Input, Discount Rate [Member] | Maximum [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Discount Rate   9.10%  
Measurement Input, Discount Rate [Member] | Arithmetic Average [Member]      
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]      
Discount Rate   8.30%