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Basis of Presentation (Details) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 USD ($) mi² retail_customers | Mar. 31, 2017 USD ($)
Basis of Presentation [Abstract] |  |
Service Area Sq Miles | mi² | 4,000 |
Incorporated Cities | 51 |
Number of Retail Customers | retail_customers | 877,000 |
Percent of State's Population | 46.00% |
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax | $ 0 | $ 1
Alternative revenue programs, net of amortization | 2 | 0
Revenues | 493 | 530
Defined Benefit Plan, Non-service Cost | 1 | $ 1
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year | $ 3 |

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