Fair Value Measurements (Schedule of Gains and Losses Included in the Level 3 Fair Value Category) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (5)	$ 97	$ (9)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets still held at the reporting date	2	6	0
Operating Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	6	4	11
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets still held at the reporting date	1	4	1
Electric Fuel and Other Energy-Related Purchases
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	(11)	97	(19)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets still held at the reporting date	1	1	0
Purchased Gas
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	0	(4)	(1)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets still held at the reporting date	$ 0	$ 1	$ (1)