Fair Value Measurements (Gains and Losses Included in Earnings in the Level 3 Fair Value Category) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (1)	$ (8)	$ 1	$ (31)
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	2	(15)	0	25
Operating revenue
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	5	(10)	12	13
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	2	(15)	0	25
Electric fuel and other energy-related purchases
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	(6)	2	(11)	(44)
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	$ 0	$ 0