Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Utility Plant and Depreciation [Line Items]
Net	$ 11,371,788	$ 10,628,138
Construction work in progress	1,019,947	816,307
Palo Verde sale leaseback, net of accumulated depreciation	113,515	117,385
Intangible assets, net of accumulated amortization	90,022	123,975
Nuclear fuel, net of accumulated amortization	119,004	123,139
Total property, plant and equipment	12,714,276	11,808,944
Electric Service
Utility Plant and Depreciation [Line Items]
Generation	7,874,898	7,336,902
Transmission	2,746,508	2,494,744
Distribution	5,738,801	5,543,561
General plant	981,681	847,025
Plant in service and held for future use	17,341,888	16,222,232
Accumulated depreciation and amortization	(5,970,100)	(5,594,094)
Net	11,371,788	10,628,138
Construction work in progress	1,019,947	816,307
Palo Verde sale leaseback, net of accumulated depreciation	113,515	117,385
Intangible assets, net of accumulated amortization	90,022	123,975
Nuclear fuel, net of accumulated amortization	119,004	123,139
Total property, plant and equipment	$ 12,714,276	$ 11,808,944