Palo Verde (Narrative) (Details) - USD ($) $ in Thousands							3 Months Ended
	Mar. 10, 2017	Oct. 31, 2016	Mar. 31, 2016	Nov. 02, 2015	Jun. 01, 2015	Oct. 31, 2014	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Jointly Owned Utility Plant Interests [Line Items]
Decommissioning Fund Investments							$ 265,857		$ 255,708
Third Party Decommission Cost Estimates Study Period, in Years							3 years
Required period in years participants need a new decommissioning study							3 years
Refund passed through to customers							$ 8,530	$ 4,104
Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Decommissioning Fund Investments							265,900
Estimated Decommissioning Costs							432,800
Decommission Cost Increase							52,100
Asset Retirement Obligation, Revision of Estimate Increase							$ 3,100
Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Settlement Agreement, Counterparty's Name							U.S. Department of Energy
Lawsuit Filing Date		Oct. 31, 2016		Nov. 02, 2015		Oct. 31, 2014
Suit Against Department of Energy for Spent Fuel | Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Settlement, Amount	$ 1,800		$ 1,900		$ 6,600
Refund passed through to customers	$ 1,400		$ 1,600		$ 5,800
Settlement Agreement Award Received, Date	Mar. 10, 2017		Mar. 31, 2016		Jun. 01, 2015
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
Minimum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered		Jul. 01, 2015		Jul. 01, 2014		Jul. 01, 2011
Maximum | Suit Against Department of Energy for Spent Fuel
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered		Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2014