Palo Verde (Narrative) (Details) - USD ($) $ in Thousands							3 Months Ended
	Mar. 31, 2018	Oct. 31, 2017	Mar. 31, 2017	Oct. 31, 2016	Aug. 18, 2014	Dec. 19, 2012	Mar. 31, 2018	Mar. 31, 2017
Jointly Owned Utility Plant Interests [Line Items]
Refund passed through to customers							$ 7,965	$ 8,530
Suit Against Department of Energy for Spent Fuel | Palo Verde Generating Station
Jointly Owned Utility Plant Interests [Line Items]
Settlement Agreement, Counterparty's Name						United States Department of Energy
Loss Contingency, Settlement Agreement, Date					August 18, 2014
Refund passed through to customers	$ 1,121		$ 1,432
Proceeds from Legal Settlements	$ 1,413		$ 1,779
Suit Against Department of Energy for Spent Fuel | Minimum
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered		July 1, 2016		July 1, 2015
Suit Against Department of Energy for Spent Fuel | Maximum
Jointly Owned Utility Plant Interests [Line Items]
Litigation, Period Covered		June 30, 2017		June 30, 2016