Utility Plant, Palo Verde and Other Jointly-Owned Utility Plant (Depreciable Assets By Each Major Class) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	$ 3,616,301	$ 3,229,255
Accumulated Depreciation	(1,329,843)	(1,266,672)
Property, Plant and Equipment, Net	2,286,458	$ 1,962,583
Intangible
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	78,309
Accumulated Depreciation	(50,097)
Property, Plant and Equipment, Net	28,212
Nuclear production | Energy Equipment
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	917,483
Accumulated Depreciation	(304,060)
Property, Plant and Equipment, Net	613,423
Steam and other | Energy Equipment
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	907,351
Accumulated Depreciation	(311,081)
Property, Plant and Equipment, Net	596,270
Total production | Energy Equipment
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	1,824,834
Accumulated Depreciation	(615,141)
Property, Plant and Equipment, Net	1,209,693
Transmission | Energy Equipment
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	459,188
Accumulated Depreciation	(256,899)
Property, Plant and Equipment, Net	202,289
Distribution | Energy Equipment
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	1,068,108
Accumulated Depreciation	(353,713)
Property, Plant and Equipment, Net	714,395
General | Energy Equipment
Property, Plant and Equipment and Finite-Lived Intangible Assets [Line Items]
Gross Plant	185,862
Accumulated Depreciation	(53,993)
Property, Plant and Equipment, Net	$ 131,869