Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	29.00%	20.00%	27.00%	12.00%
Production tax credits	$ 50	$ 84	$ 112	$ 163
Deferred income tax benefit associated with convertible investment tax credits	10	1	23	8
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