Financial and Other Derivative Instruments (Effect of Derivatives not Designated as Hedging Instruments on the Consolidated Statement of Operations) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	$ 16	$ (19)	$ 127	$ (65)
Natural Gas | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	20	(30)	77	(86)
Natural Gas | Fuel, purchased power, and gas — non-utility
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	(15)	(9)	46	32
Electricity | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	13	19	6	(5)
Other | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	(1)	0	(1)	(2)
Foreign currency exchange contracts | Operating Revenues — Non-utility operations
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in Income on Derivatives	$ (1)	$ 1	$ (1)	$ (4)