Basis of Presentation (Details) retail_customers in Thousands, mi² in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) mi² retail_customers	Mar. 31, 2018 USD ($)
Basis of Presentation [Abstract]
Service Area Sq Miles | mi²	4
Incorporated Cities	51
Number of Retail Customers | retail_customers	887
Percent of state population	46.00%
Alternative revenue programs, net of amortization	$ 3	$ (2)
Reclassification of stranded tax effects due to Tax Reform	2
Revenues	573	$ 493
AOCI Attributable to Parent [Member]
Reclassification of stranded tax effects due to Tax Reform	2
Property, Plant and Equipment [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(178)
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	44
Accounting Standards Update 2016-02 [Member] | Property, Plant and Equipment [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	2
Building [Member] | Property, Plant and Equipment [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	131
Assets Held under Capital Leases [Member] | Property, Plant and Equipment [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 49