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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ (9)	$ (2)	$ 1	$ 98
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	1	2
Operating revenue
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	0	3	2	(7)
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	3	1	4
Purchased Gas
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	0	(3)	0	(4)
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	(2)	0	(2)
Electric fuel and other energy-related purchases
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	(9)	(2)	(1)	109
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	$ 0	$ 0