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, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 8,101	$ 7,297	$ 6,409
Future Amortization Expense, 2020	8,403
Future Amortization Expense, 2021 (estimated)	7,667
Future Amortization Expense, 2022 (estimated)	7,063
Future Amortization Expense, 2023 (estimated)	6,245
Future Amortization Expense, 2024 (estimated)	$ 4,152
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