Income Taxes (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Income Tax Expense to Federal Tax Rate [Line Items]
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	$ 768	$ 704	$ 711
Income tax expense at 35% statutory rate	269	246	249
Investment tax credits	(6)	(6)	(6)
Depreciation	3	3	3
Employee Stock Ownership Plan dividends	(3)	(3)	(3)
Domestic production activities deduction	(16)	(6)	(6)
State and other income taxes, net of federal benefit	40	39	40
Other, net	(5)	(6)	(7)
Income Tax Expense	$ 282	$ 267	$ 270
Effective Income Tax Rate	36.70%	38.00%	38.00%