Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Utility Plant and Depreciation [Line Items]
Net	$ 11,669,526	$ 11,371,788
Construction work in progress	1,291,498	1,019,947
Palo Verde sale leaseback, net of accumulated depreciation	109,645	113,515
Intangible assets, net of accumulated amortization	257,189	90,022
Nuclear fuel, net of accumulated amortization	117,408	119,004
Total property, plant and equipment	13,445,266	12,714,276
Electric Service
Utility Plant and Depreciation [Line Items]
Generation	7,963,998	7,874,898
Transmission	2,836,578	2,746,508
Distribution	6,025,856	5,738,801
General plant	971,629	981,681
Plant in service and held for future use	17,798,061	17,341,888
Accumulated depreciation and amortization	(6,128,535)	(5,970,100)
Net	11,669,526	11,371,788
Construction work in progress	1,291,498	1,019,947
Palo Verde sale leaseback, net of accumulated depreciation	109,645	113,515
Intangible assets, net of accumulated amortization	257,189	90,022
Nuclear fuel, net of accumulated amortization	117,408	119,004
Total property, plant and equipment	$ 13,445,266	$ 12,714,276