Basis of Presentation (Details) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) mi² retail_customers shares	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Basis of Presentation [Abstract]
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	$ 72	$ 61
Service Area Sq Miles | mi²	4,000
Incorporated Cities	51
Number of Retail Customers | retail_customers	865,000
Service Area Population | shares	1.9
Percent of State's Population	46.00%
Decoupling Mechanism Deferrals, Net	$ 9	4
Repayments of Long-term Capital Lease Obligations		1
Other Comprehensive Income	1
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	(1)	0
Unamortized Debt Issuance Expense	$ 11	$ 1	$ 11