Basis of Presentation (Details) retail_customers in Thousands, mi² in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) mi² retail_customers	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) mi² retail_customers	Jun. 30, 2017 USD ($)
Basis of Presentation [Abstract]
Service Area Sq Miles | mi²	4		4
Incorporated Cities	51		51
Number of Retail Customers | retail_customers	883		883
Percent of state population	46.00%		46.00%
Defined Benefit Plan, Non-service Cost	$ 2	$ 1	$ 3	$ 2
Alternative revenue programs, net of amortization	0	0	2	0
Revenues	449	$ 449	942	$ 979
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	$ 1		$ 2