Basis of Presentation (Details) shares in Millions, $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2016 USD ($) mi² retail_customers shares	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Basis of Presentation [Abstract]
Regulatory deferral of settled derivative instruments					$ 2
Service Area Sq Miles | mi²				4,000
Incorporated Cities				52
Number of Retail Customers | retail_customers				859,497
Service Area Population | shares				1.8
Percent of State's Population				46.00%
Other Comprehensive Income		$ 0		$ 0
Unamortized Debt Issuance Expense				$ 12		$ 11