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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
Dec. 31, 2011
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]          
Federal statutory income tax rate 35.00% 35.00% 35.00% 35.00%  
Federal and state income tax credits (14.00%) (10.00%) (12.00%) (10.00%)  
State income tax, net of federal income tax benefit 2.00% 1.00% 2.00% 1.00%  
Income tax effect of foreign income (3.00%) (2.00%) (3.00%) (2.00%)  
Income tax method change (6.00%) 0.00% (2.00%) 0.00%  
Effects of ratemaking (3.00%) (1.00%) (3.00%) (1.00%)  
Other, net 1.00% 2.00% 1.00% 2.00%  
Effective income tax rate 12.00% 25.00% 18.00% 25.00%  
Production Tax Credit Carryforwards [Abstract]          
Years eligible for federal renewable energy production tax credit     10 years    
Income tax method change [Abstract]          
Income tax method change $ 18   $ 18    
Related Party Income Tax Recievable (Payable) [Abstract]          
Income tax receivable from (payable to) Berkshire Hathaway $ (168)   $ (168)   $ 456