Fair Value Measurements (Gains and Losses Included in Earnings in the Level 3 Fair Value Category) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (10)	$ (5)	$ (17)	$ 10
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
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	$ (10)	$ (5)	(17)	8
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)