Electric utility subsidiary (Details 5) (Operating revenues, Customer concentration, Various federal government agencies, USD $) In Millions, unless otherwise specified	12 Months Ended
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

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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