Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Utility Plant and Depreciation [Line Items]
Depreciation and amortization	$ 582,354	$ 534,118	$ 485,829
Net	12,370,614	11,669,526
Construction work in progress	1,170,062	1,291,498
Palo Verde sale leaseback, net of accumulated depreciation	105,775	109,645
Intangible assets, net of accumulated amortization	262,902	257,189
Nuclear fuel, net of accumulated amortization	120,217	117,408
Total property, plant and equipment	14,029,570	13,445,266
Electric Service
Utility Plant and Depreciation [Line Items]
Generation	8,285,514	7,963,998
Transmission	3,033,579	2,836,578
Distribution	6,378,345	6,025,856
General plant	1,039,190	971,629
Plant in service and held for future use	18,736,628	17,798,061
Accumulated depreciation and amortization	(6,366,014)	(6,128,535)
Net	12,370,614	11,669,526
Construction work in progress	1,170,062	1,291,498
Palo Verde sale leaseback, net of accumulated depreciation	105,775	109,645
Intangible assets, net of accumulated amortization	262,902	257,189
Nuclear fuel, net of accumulated amortization	120,217	117,408
Total property, plant and equipment	$ 14,029,570	$ 13,445,266