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Basis of Presentation (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2018
USD ($)
mi²
shares
retail_customers
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Basis of Presentation [Abstract]      
Unrealized Gain (Loss) on Securities | $   $ 2 $ 5
Service Area Sq Miles | mi² 4,000    
Incorporated Cities 51    
Number of Retail Customers | retail_customers 885,000    
Service area population 1,900,000    
Percent of State's Population 46.00%    
Entity Number of Employees 2,967    
Number of Union Employees 802    
Number of Union Employees Subject to Agreement A 747    
Number of Union Employees Subject to Agreement B 55    
Prior Period Reclassification Adjustment | $ $ 50 $ (5) $ (2)