Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Narrative) (Details) $ in Thousands											12 Months Ended
	Oct. 31, 2016 USD ($)	Jul. 06, 2016 USD ($)	Mar. 31, 2016 USD ($)	Nov. 02, 2015 USD ($)	Jun. 01, 2015 USD ($)	Oct. 31, 2014 USD ($)	Oct. 08, 2014 USD ($)	Aug. 18, 2014 USD ($)	Mar. 31, 2013 USD ($)	Dec. 19, 2012	Dec. 31, 2016 USD ($) utilityplantunit	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Apr. 21, 2011
Jointly Owned Utility Plant Interests [Line Items]
Proceeds from sale of property, plant, and equipment											$ 4,841	$ 721	$ 2,395
Decommissioning Fund Investments											$ 255,708	239,035
Third Party Decommission Cost Estimates Study Period, in Years											3 years
Estimated Decommissioning Costs									$ 380,700
Decommission cost increase									$ 23,300
Asset Retirement Obligation, Revision of Estimate											$ 0	0	3,561
Required period in years participants need a new decommissioning study											3 years
Retrospective Premium Adjustments Due to Nuclear Facility Losses											$ 12,900
Asset Retirement Obligation											81,800	81,621	74,577	$ 65,214
Refund passed through to customers											$ (14,891)	$ 13,344	$ (3,121)
Four Corners Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Proceeds from sale of property, plant, and equipment		$ 4,200
Jointly Owned Utility Plant, Proportionate Ownership Share		7.00%
Property, Plant and Equipment, Disposals		$ 32,000
Disposal Date		Jul. 06, 2016
Asset Retirement Obligation		$ 7,000
Mine Reclamation and Closing Liability, Noncurrent		19,500
Public Utilities, Property, Plant and Equipment, Amount of Disposal Adjustments		$ 1,300
Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Number Of Power Generation Units											3
Number of Units With Renewed Operating Licenses															3
Asset Retirement Obligation, Revision of Estimate														$ 1,900
Maximum Assesment Per Reactor Nuclear Energy Incident											$ 60,400
Maximum Assesment Per Reactor Nuclear Energy Incident Annual Limit											$ 9,000
Jointly Owned Utility Plant, Proportionate Ownership Share											15.80%
Jointly Owned Utility Plant, Number of Units | utilityplantunit											3
Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Nuclear Energy Hazards Full Limit Of Liability Under Federal Law											$ 13,400,000
Nuclear Energy Hazards Potential Liability											450,000
Maximum Assesment Per Reactor Nuclear Energy Incident											127,300
Maximum Assesment Per Reactor Nuclear Energy Incident Annual Limit											19,000
Aggregate Amount of Insurance for Decontamination and Property Damage											2,750,000
Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Settlement Agreement, Counterparty's Name										United States Department of Energy
Lawsuit Filing Date	October 31, 2016			November 2, 2015		October 31, 2014				December 19, 2012
Loss Contingency, Settlement Agreement, Date								August 18, 2014
Suit Against Department of Energy for Spent Fuel | Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Settlement Agreement Award Received, Date					Jun. 01, 2015		Oct. 08, 2014
Settlement, Amount			$ 1,900		$ 6,600		$ 9,100
Litigation, Claim Amount	$ 1,800
Refund passed through to customers			$ 1,600		$ 5,800		$ 7,900
Suit Against Department of Energy for Spent Fuel | Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Settlement, Amount	$ 11,300			$ 12,000		$ 42,000		$ 57,400
Minimum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered	July 1, 2015			Jul. 01, 2014		Jul. 01, 2011				Jan. 01, 2007
Maximum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered	June 30, 2016			Jun. 30, 2015		Jun. 30, 2014				Jun. 30, 2011
Limited amount of coverage | Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Aggregate Amount of Insurance for Decontamination and Property Damage											$ 2,250,000
PUCT | Four Corners Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Disposal Date											Jul. 06, 2016
Mine Reclamation and Closing Liability, Noncurrent											$ 7,300
Public Utilities, Regulatory Rate Making Impact
Jointly Owned Utility Plant Interests [Line Items]
Decrease in depreciation expense											$ 10,900