Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	Dec. 31, 2020 USD ($) $ / MWh $ / MMBTU	Dec. 31, 2019 USD ($) $ / MMBTU $ / MWh
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 567	$ 775
Derivative Liabilities	(533)	(770)
Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	233	355
Derivative Liabilities	(223)	(351)
Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	180	306
Derivative Liabilities	$ (168)	$ (298)
Level 3 | Discounted Cash Flow | Forward basis price | Natural Gas | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.86)	(1.78)
Level 3 | Discounted Cash Flow | Forward basis price | Natural Gas | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	2.50	5.78
Level 3 | Discounted Cash Flow | Forward basis price | Natural Gas | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.07)	(0.09)
Level 3 | Discounted Cash Flow | Forward basis price | Electricity | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(9)	(10)
Level 3 | Discounted Cash Flow | Forward basis price | Electricity | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	6	6
Level 3 | Discounted Cash Flow | Forward basis price | Electricity | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	0	0
Recurring | Level 3
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 116	$ 160
Derivative Liabilities	(118)	(156)
Recurring | Level 3 | Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	60	74
Derivative Liabilities	(76)	(89)
Recurring | Level 3 | Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	52	83
Derivative Liabilities	$ (42)	$ (67)