Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Utility Plant and Depreciation [Line Items]
Net	$ 13,198,435	$ 12,370,614
Construction work in progress	808,133	1,170,062
Palo Verde sale leaseback, net of accumulated depreciation	101,906	105,775
Intangible assets, net of accumulated amortization	290,564	262,902
Nuclear fuel, net of accumulated amortization	123,500	120,217
Total property, plant and equipment	14,522,538	14,029,570
Electric Service
Utility Plant and Depreciation [Line Items]
Generation	8,916,872	8,285,514
Transmission	3,095,907	3,033,579
Distribution	6,690,697	6,378,345
General plant	1,132,816	1,039,190
Plant in service and held for future use	19,836,292	18,736,628
Accumulated depreciation and amortization	(6,637,857)	(6,366,014)
Net	13,198,435	12,370,614
Construction work in progress	808,133	1,170,062
Palo Verde sale leaseback, net of accumulated depreciation	101,906	105,775
Intangible assets, net of accumulated amortization	290,564	262,902
Nuclear fuel, net of accumulated amortization	123,500	120,217
Total property, plant and equipment	$ 14,522,538	$ 14,029,570