Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	Dec. 31, 2019 USD ($) $ / MMBTU $ / MWh	Dec. 31, 2018 USD ($) $ / MMBTU $ / MWh
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 775	$ 663
Derivative Liabilities	(770)	(669)
Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	355	349
Derivative Liabilities	(351)	(380)
Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	306	303
Derivative Liabilities	$ (298)	$ (285)
Level 3 | Discounted Cash Flow | Forward basis price | Natural Gas | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(1.78)	(2.15)
Level 3 | Discounted Cash Flow | Forward basis price | Natural Gas | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	5.78	5.59
Level 3 | Discounted Cash Flow | Forward basis price | Natural Gas | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MMBTU	(0.09)	(0.10)
Level 3 | Discounted Cash Flow | Forward basis price | Electricity | Minimum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	(10)	(7)
Level 3 | Discounted Cash Flow | Forward basis price | Electricity | Maximum
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	6	9
Level 3 | Discounted Cash Flow | Forward basis price | Electricity | Weighted Average
Unobservable Input Valuation Techniques [Line Items]
Forward basis price | $ / MWh	0	1
Recurring | Level 3
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	$ 160	$ 126
Derivative Liabilities	(156)	(170)
Recurring | Level 3 | Natural Gas
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	74	63
Derivative Liabilities	(89)	(112)
Recurring | Level 3 | Electricity
Unobservable Input Valuation Techniques [Line Items]
Derivative Assets	83	56
Derivative Liabilities	$ (67)	$ (58)