Basis of Presentation (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) mi² retail_customers	Sep. 30, 2016 USD ($)
Basis of Presentation [Abstract]
Service Area Sq Miles | mi²	4,000
Incorporated Cities	51
Number of Retail Customers | retail_customers	873,000
Percent of State's Population	46.00%
Decoupling Mechanism Deferrals, Net	$ (15)	$ (4)
Defined Benefit Plan, Non-service Cost	$ 2