Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	Jun. 30, 2021 USD ($) MWh MMBTU	Dec. 31, 2020 USD ($) MMBTU MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 1,041	$ 567
Derivative Liabilities	(1,216)	(533)
Natural gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	336	233
Derivative Liabilities	(528)	(223)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	330	180
Derivative Liabilities	$ (325)	$ (168)
Level 3 | Natural gas | Forward basis price | Minimum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(1.41)	(0.86)
Level 3 | Natural gas | Forward basis price | Maximum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	2.47	2.50
Level 3 | Natural gas | Forward basis price | Weighted Average | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(0.09)	(0.07)
Level 3 | Electricity | Forward basis price | Minimum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	(12)	(9)
Level 3 | Electricity | Forward basis price | Maximum | Discounted Cash Flow
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	5	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	$ 136	$ 116
Derivative Liabilities	(351)	(118)
Recurring | Level 3 | Natural gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	46	60
Derivative Liabilities	(227)	(76)
Recurring | Level 3 | Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	75	52
Derivative Liabilities	$ (124)	$ (42)