Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Amortization of Intangible Plant Assets - Software) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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]
Decrease in depreciation expense	$ 1,800
Expected decrease in 2016 annual depreciation expense	3,600
Finite-Lived Intangible Assets, Amortization Expense		$ 6,482	$ 8,051	$ 7,683
Future Amortization Expense, 2016 (estimated)	5,022	5,022
Future Amortization Expense, 2017 (estimated)	4,602	4,602
Future Amortization Expense, 2018 (estimated)	3,818	3,818
Future Amortization Expense, 2019 (estimated)	3,382	3,382
Future Amortization Expense, 2020 (estimated)	$ 2,935	$ 2,935
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