Income Taxes Components of Income Tax Expense (Benefit) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Disclosure [Abstract]
Current federal	$ (112)	$ (151)	$ (498)
Current state	(3)	(10)	(1)
Total current income tax expense (benefit)	(115)	(161)	(499)
Deferred federal	283	338	684
Deferred state	33	40	30
Total deferred income tax expense (benefit)	316	378	714
Investment tax credits	(4)	(4)	(4)
Total income tax expense	$ 197	$ 213	$ 211