Fair Value Measurements - Schedule of Fair Value of the Level 3 Assets and Liabilities by Major Contract Type and the Significant Unobservable Inputs Used in the Valuations (Details) - Level 3	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	$ 727,000,000	$ 841,000,000
Liabilities	(1,674,000,000)	(1,593,000,000)
Total	(947,000,000)	(752,000,000)
Electricity purchases and sales | Income Approach
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	467,000,000	606,000,000
Liabilities	(1,457,000,000)	(1,399,000,000)
Total	(990,000,000)	(793,000,000)
Electricity purchases and sales | Minimum | Income Approach | Hourly price curve shape
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	0	0
Electricity purchases and sales | Minimum | Income Approach | Illiquid delivery periods for hub power prices
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	25	25
Electricity purchases and sales | Minimum | Income Approach | Market Heat Rates
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	25	30
Electricity purchases and sales | Maximum | Income Approach | Hourly price curve shape
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	95	95
Electricity purchases and sales | Maximum | Income Approach | Illiquid delivery periods for hub power prices
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	135	140
Electricity purchases and sales | Maximum | Income Approach | Market Heat Rates
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	135	150
Electricity purchases and sales | Arithmetic Average | Income Approach | Hourly price curve shape
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	48	48
Electricity purchases and sales | Arithmetic Average | Income Approach | Illiquid delivery periods for hub power prices
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	80	83
Electricity purchases and sales | Arithmetic Average | Income Approach | Market Heat Rates
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	80	90
Options | Option Pricing Model
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	3,000,000	6,000,000
Liabilities	(173,000,000)	(139,000,000)
Total	$ (170,000,000)	$ (133,000,000)
Options | Minimum | Option Pricing Model | Natural gas to power correlation
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input	0.15	0.10
Options | Minimum | Option Pricing Model | Power and natural gas volatility
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input	0.05	0.05
Options | Maximum | Option Pricing Model | Natural gas to power correlation
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input	1	1
Options | Maximum | Option Pricing Model | Power and natural gas volatility
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input	9.70	7.10
Options | Arithmetic Average | Option Pricing Model | Natural gas to power correlation
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input	0.58	0.55
Options | Arithmetic Average | Option Pricing Model | Power and natural gas volatility
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input	4.88	3.58
Financial transmission rights/Congestion revenue rights | Market Approach
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	$ 231,000,000	$ 190,000,000
Liabilities	(33,000,000)	(25,000,000)
Total	198,000,000	165,000,000
Financial transmission rights/Congestion revenue rights | Minimum | Market Approach | Illiquid price differences between settlement points
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	(12)	(35)
Financial transmission rights/Congestion revenue rights | Maximum | Market Approach | Illiquid price differences between settlement points
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	25	20
Financial transmission rights/Congestion revenue rights | Arithmetic Average | Market Approach | Illiquid price differences between settlement points
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	6.5	(8)
Natural gas | Income Approach
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	16,000,000	29,000,000
Liabilities	(11,000,000)	(30,000,000)
Total	5,000,000	(1,000,000)
Natural gas | Minimum | Income Approach | Natural gas basis
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	(1)	0
Natural gas | Minimum | Income Approach | Illiquid delivery periods
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	3	0
Natural gas | Maximum | Income Approach | Natural gas basis
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	14	10
Natural gas | Maximum | Income Approach | Illiquid delivery periods
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	5	5
Natural gas | Arithmetic Average | Income Approach | Natural gas basis
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	6	5
Natural gas | Arithmetic Average | Income Approach | Illiquid delivery periods
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative asset (liability), measurement input amount	4	2
Other
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets	10,000,000	10,000,000
Liabilities	0	0
Total	$ 10,000,000	$ 10,000,000