Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Narrative) (Details) $ in Thousands					12 Months Ended
	Oct. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) utilityplantunit	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 21, 2011
Jointly Owned Utility Plant Interests [Line Items]
Decommissioning Fund Investments					$ 325,998	$ 276,905
Estimated Decommissioning Costs								$ 432,800
Decommissioning Costs Increase (Decrease)								52,100
Required period in years participants need a new decommissioning study					3 years
Retrospective Premium Adjustments Due to Nuclear Facility Losses					$ 13,800
Refund passed through to customers					7,368	$ 4,822	$ 17,093
Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Nuclear Energy Hazards Full Limit Of Liability Under Federal Law					13,900,000
Nuclear Energy Hazards Potential Liability					450,000
Maximum Assesment Per Reactor Nuclear Energy Incident					137,600
Maximum Assesment Per Reactor Nuclear Energy Incident Annual Limit					20,500
Aggregate Amount of Insurance for Decontamination and Property Damage					2,800,000
Palo Verde Generation Station
Jointly Owned Utility Plant Interests [Line Items]
Number of Units With Renewed Operating Licenses									3
Asset Retirement Obligation, Revision of Estimate								$ 3,500
Maximum Assesment Per Reactor Nuclear Energy Incident					65,200
Maximum Assesment Per Reactor Nuclear Energy Incident Annual Limit					$ 9,700
Jointly Owned Utility Plant, Proportionate Ownership Share					15.80%
Jointly Owned Utility Plant, Number of Units | utilityplantunit					3
Suit Against Department of Energy for Spent Fuel | Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Claim Amount	$ 16,000
Suit Against Department of Energy for Spent Fuel | Palo Verde Generation Station
Jointly Owned Utility Plant Interests [Line Items]
Proceeds from Legal Settlements		$ 1,604	$ 1,413	$ 1,779
Refund passed through to customers		$ 1,005	$ 1,121	$ 1,432
Limited amount of coverage | Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Aggregate Amount of Insurance for Decontamination and Property Damage					$ 2,300,000
Pending Litigation | Suit Against Department of Energy for Spent Fuel | Palo Verde Generation Station
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Claim Amount	$ 2,500