Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	Sep. 30, 2021 USD ($) MMBTU MWh	Dec. 31, 2020 USD ($) MMBTU MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 2,340	$ 567
Derivative Liabilities	(2,557)	(533)
Natural gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	778	233
Derivative Liabilities	(1,055)	(223)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	952	180
Derivative Liabilities	$ (916)	$ (168)
Level 3 | Natural gas | Forward basis price | Minimum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(1.33)	(0.86)
Level 3 | Natural gas | Forward basis price | Maximum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	3.53	2.50
Level 3 | Natural gas | Forward basis price | Weighted Average | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(0.04)	(0.07)
Level 3 | Electricity | Forward basis price | Minimum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	(8)	(9)
Level 3 | Electricity | Forward basis price | Maximum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	10	6
Level 3 | Electricity | Forward basis price | Weighted Average | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	(1)	0
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 310	$ 116
Derivative Liabilities	(673)	(118)
Recurring | Level 3 | Natural gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	53	60
Derivative Liabilities	(333)	(76)
Recurring | Level 3 | Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	245	52
Derivative Liabilities	$ (340)	$ (42)