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Basis of Presentation (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2017
mi²
shares
retail_customers
Basis of Presentation [Abstract]      
Service Area Sq Miles | mi²     4,000
Incorporated Cities     51
Number of Retail Customers | retail_customers     875,000
Service area population     1,900,000
Percent of State's Population     46.00%
Entity Number of Employees     2,906
Number of Union Employees     785
Number of Union Employees Subject to Agreement A     732
Number of Union Employees Subject to Agreement B     53
Cash received to be returned to customers pursuant to the Residential Exchange Program | $ $ 6 $ 1  
Contribution to the voluntary employees’ beneficiary association trust | $ 2 4  
Regulatory deferral of settled derivative instruments | $ $ 2 $ 2