Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Summary of Jointly Owned Utility Plants) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) utilityplantunit	Dec. 31, 2016 USD ($)
Palo Verde Participants
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Plant, Number of Participants	6
Jointly Owned Nuclear Power Plant
Jointly Owned Utility Plant Interests [Line Items]
Jointly Owned Utility Plant, Proportionate Ownership Share	15.80%
Jointly Owned Utility Plant, Number of Units | utilityplantunit	3
Electric plant in service	$ 994,075	$ 948,382
Accumulated depreciation	(338,699)	(320,000)
Construction work in progress	40,946	50,598
Total	696,322	678,980
Other Jointly Owned Utlity Plant
Jointly Owned Utility Plant Interests [Line Items]
Electric plant in service	97,603	97,652
Accumulated depreciation	(72,822)	(74,408)
Construction work in progress	1,014	1,895
Total	$ 25,795	$ 25,139