Fair Value (Unobservable Inputs related to Level 3 Assets and Liabilities) (Details) $ in Millions	Mar. 31, 2021 USD ($) MWh MMBTU	Dec. 31, 2020 USD ($) MWh MMBTU
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	$ 560	$ 567
Derivative Liabilities	(652)	(533)
Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	63	116
Derivative Liabilities	(170)	(118)
Natural gas
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	163	233
Derivative Liabilities	(246)	(223)
Natural gas | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	42	60
Derivative Liabilities	(131)	(76)
Electricity
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	138	180
Derivative Liabilities	(150)	(168)
Electricity | Recurring | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Assets	20	52
Derivative Liabilities	$ (39)	$ (42)
Forward basis price | Natural gas | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(1.10)	(0.86)
Forward basis price | Natural gas | Maximum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	2.45	2.50
Forward basis price | Natural gas | Weighted Average | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MMBTU	(0.10)	(0.07)
Forward basis price | Electricity | Minimum | Discounted Cash Flow | Level 3
Unobservable Inputs Valuation Techniques [Line Items]
Forward basis price | MWh	(9)	(9)
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