Fair Value Measurements (Schedule of Gains and Losses Included in the Level 3 Fair Value Category) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments Gain Loss [Line Items]
Total gains (losses) included in earnings	$ (11)	$ (7)	$ (36)	$ (23)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date	1		1
Operating Revenue
Derivative Instruments Gain Loss [Line Items]
Total gains (losses) included in earnings	1	0	1	0
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date	1		1
Electric Fuel and Other Energy-Related Purchases
Derivative Instruments Gain Loss [Line Items]
Total gains (losses) included in earnings	(12)	$ (7)	(37)	$ (23)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date	$ 0		$ 0