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Electric utility segment Major Customers (Details) - Operating revenues - Customer concentration - Various federal government agencies - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Major customers |  |  |
Operating revenues percentage | 12.00% | 11.00% | 11.00%
Operating revenues amount | $ 350 | $ 340 | $ 349

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