Financial and Other Derivative Instruments (Effect of Derivatives not Designated as Hedging Instruments on the Consolidated Statement of Operations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	$ (83)	$ 55	$ (81)
Natural gas | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	(70)	44	(42)
Natural gas | Fuel, purchased power, gas, and other — non-utility
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	20	(5)	(94)
Electricity | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	91	44	49
Environmental & Other | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	(118)	(26)	(1)
Foreign currency exchange contracts | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	$ (6)	$ (2)	$ 7