Regulatory Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Regulatory Assets
Regulatory Assets, Current	$ 494	$ 378
Regulatory Asset, Long-term	5,466	4,347
Total Regulatory Assets	5,960	4,725
Regulatory assets, energy derivatives, low range of contract expiration (in years)	1 year
Regulatory assets, energy derivatives, high range of contract expiration (in years)	12 years
Regulatory assets related to deferred income taxes, recovery period, low range (in years)	1 year
Regulatory assets related to deferred income taxes, recovery period, high range (in years)	45 years
Regulatory Liabilities
Regulatory Liabilities, Current	670	738
Regulatory Liabilities, Long-term	4,670	4,524
Total Regulatory Liabilities	5,340	5,262
Regulatory Balancing Accounts, Assets
Regulatory Assets
Regulatory Assets, Current	223	213
Regulatory Asset, Long-term	69	56
Energy Derivatives
Regulatory Assets
Regulatory Assets, Current	264	162
Regulatory Asset, Long-term	487	177
Other
Regulatory Assets
Regulatory Assets, Current	7	3
Regulatory Asset, Long-term	298	280
Regulatory Liabilities
Regulatory Liabilities, Current	9	5
Regulatory Liabilities, Long-term	4	0
Deferred Income Taxes - Net
Regulatory Assets
Regulatory Asset, Long-term	2,020	1,855
Pensions and Other Postretirement Benefits
Regulatory Assets
Regulatory Asset, Long-term	1,703	1,097
Unamortized Investment - Net
Regulatory Assets
Regulatory Asset, Long-term	484	460
Return rate earned on assets included in rate base (as a percent)	8.75%	8.75%
Unamortized Loss on Reacquired Debt
Regulatory Assets
Regulatory Asset, Long-term	249	268
Low end of the range of remaining original amortization (in years)	1 year
High end of the range of remaining original amortization (in years)	27 years
Nuclear-Related Investment - Net
Regulatory Assets
Regulatory Asset, Long-term	156	154
Costs of Removal
Regulatory Liabilities
Regulatory Liabilities, Long-term	2,697	2,623
Asset Retirement Obligations
Regulatory Liabilities
Regulatory Liabilities, Long-term	1,105	1,099
Regulatory Balancing Accounts, Liabilities
Regulatory Liabilities
Regulatory Liabilities, Current	661	733
Regulatory Liabilities, Long-term	$ 864	$ 802