Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	27.00%	28.00%	27.00%	18.00%
Production tax credits	$ 35	$ 45	$ 147	$ 208
Deferred income tax benefit associated with convertible investment tax credits	14	9	37	17
Deferred state tax benefit related to recent state tax law changes				64
Consolidated effect of reversal of valuation allowance related to certain state investment tax credits				$ 26