Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Amortization of Intangible Plant Assets - Software) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Number of years amortization expense prior reporting year	3 years
Number of years amortization expense after reporting year	5 years
Intangible Plant Software
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Amortization Expense	$ 6,409	$ 5,302	$ 6,482
Future Amortization Expense, 2018 (estimated)	6,835
Future Amortization Expense, 2019 (estimated)	6,485
Future Amortization Expense, 2020 (estimated)	6,048
Future Amortization Expense, 2021 (estimated)	5,128
Future Amortization Expense, 2022 (estimated)	$ 4,328
Intangible Plant Software | Minimum
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Useful Life	3 years
Intangible Plant Software | Maximum
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Useful Life	15 years