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Fair Value of Financial Instruments - Significant Unobservable Inputs (Details) - Level 3 - Fair Value, Recurring
$ in Millions
Sep. 30, 2025
USD ($)
$ / MWh
MWh
$ / BTU
Low | Measurement Input, Commodity Forward Price | Valuation Technique, Option Model  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / BTU 1.21
High | Measurement Input, Commodity Forward Price | Valuation Technique, Option Model  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / BTU 24.08
Weighted Average | Measurement Input, Commodity Forward Price | Valuation Technique, Option Model  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / BTU 3.92
Long-term Power Commodity Contracts | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Assets | $ $ 0
Liabilities | $ $ 336
Long-term Power Commodity Contracts | Low | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / MWh 29.89
Long-term Power Commodity Contracts | High | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / MWh 90.24
Long-term Power Commodity Contracts | Weighted Average | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / MWh 47.79
Heat Rate Call Option Commodity Contracts | Measurement Input, Commodity Forward Price | Valuation Technique, Option Model  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Assets | $ $ 12
Liabilities | $ $ 0
Heat Rate Call Option Commodity Contracts | Low | Measurement Input, Commodity Forward Price | Valuation Technique, Option Model  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / MWh (19.74)
Heat Rate Call Option Commodity Contracts | High | Measurement Input, Commodity Forward Price | Valuation Technique, Option Model  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / MWh 777.56
Heat Rate Call Option Commodity Contracts | Weighted Average | Measurement Input, Commodity Forward Price | Valuation Technique, Option Model  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | $ / MWh 47.27
Other Financial Instruments | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Assets | $ $ 7
Liabilities | $ $ 0
Other Financial Instruments | Low | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | MWh 60,047
Other Financial Instruments | High | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | MWh 120,094
Other Financial Instruments | Weighted Average | Measurement Input, Commodity Forward Price | Valuation Technique, Discounted Cash Flow  
Fair Value Measurement Inputs and Valuation Techniques [Line Items]  
Derivative asset/liability measurement input | MWh 108,791