Basis of Presentation (Details) retail_customers in Thousands, mi² in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) mi² retail_customers	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) mi² retail_customers	Sep. 30, 2017 USD ($)
Basis of Presentation [Abstract]
Service Area Sq Miles | mi²	4		4
Incorporated Cities	51		51
Number of Retail Customers | retail_customers	885		885
Percent of state population	46.00%		46.00%
Alternative revenue programs, net of amortization	$ 0	$ 0	$ (2)	$ 0
Defined Benefit Plan, Non-service Cost	1	1	4	3
Revenues	525	$ 515	1,467	$ 1,494
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year	$ 3		3
Pension and Other Postretirement Plans Costs [Member]
Defined Benefit Plan, Non-service Cost			$ 2