Electric utility segment - Major Customers (Details) - Operating revenues - Customer concentration - Various federal government agencies - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Major customers
Operating revenues percentage	11.00%	11.00%	11.00%
Operating revenues amount	$ 273	$ 239	$ 226

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.

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