Fair Value Measurements (Net Change in the Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Deferred issuance expenses	$ 2	$ 2
Total gains (losses) included in earnings	(15)	(77)	43
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	42	22	(4)
Operating Revenue
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	35	(32)	(4)
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	42	22	(4)
Electric Fuel and Energy Purchases
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	(50)	(45)	51
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	0	0
Purchased Gas
Derivative Instruments, Gain (Loss) [Line Items]
Total gains (losses) included in earnings	0	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 still held at the reporting date	$ 0	$ 0	$ 0