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Electric utility subsidiary (Details 5) (Operating revenues, Customer concentration, Various federal government agencies, USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Operating revenues | Customer concentration | Various federal government agencies |  |  |
Major customers |  |  |
Operating revenues percentage | 11.00% | 10.00% | 10.00%
Operating revenues amount | $ 316 | $ 242 | $ 199

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