Fair Value Measurements (Gains and Losses Included in Earnings in the Level 3 Fair Value Category) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (7)	$ (9)	$ (23)	$ 1
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	0	0	0	2
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		1
Electric Fuel and Other Energy-Related Purchases
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (7)	(9)	$ (23)	(1)
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		$ 0