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Fair Value of Financial Instruments - Significant Unobservable Inputs (Details) - Fair Value, Recurring - Energy Related Derivative - Level 3
$ in Millions
Sep. 30, 2020
USD ($)
$ / MWh
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset | $ $ 0
Derivative liability | $ $ (17)
Low | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative liability measurement input 9.21
High | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative liability measurement input 37.46
Weighted Average | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative liability measurement input 16.32