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
	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	$ 15	$ 110
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
Operating revenue
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	2	(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	1
Electric fuel and other energy-related purchases
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	13	120
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