Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	Sep. 30, 2020 USD ($) MWh MMBTU	Dec. 31, 2019 USD ($) MMBTU MWh
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 580	$ 775
Derivative Liabilities	(564)	(770)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	83	160
Derivative Liabilities	(110)	(156)
Natural gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	240	355
Derivative Liabilities	(265)	(351)
Natural gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	42	74
Derivative Liabilities	(81)	(89)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	137	306
Derivative Liabilities	(126)	(298)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	34	83
Derivative Liabilities	$ (29)	$ (67)
Forward basis price | Natural gas | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(1.10)	(1.78)
Forward basis price | Natural gas | Maximum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	2.30	5.78
Forward basis price | Natural gas | Weighted Average | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(0.06)	(0.09)
Forward basis price | Electricity | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	(9)	(10)
Forward basis price | Electricity | Maximum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	6	6
Forward basis price | Electricity | Weighted Average | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	(1)	0