Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Utility Plant and Depreciation [Line Items]
Net	$ 13,727,575	$ 13,198,435
Construction work in progress	937,384	808,133
Intangible assets, net of accumulated amortization	282,570	290,564
Nuclear fuel, net of accumulated amortization	113,645	123,500
Total property, plant and equipment	15,159,210	14,522,538
Electric Service
Utility Plant and Depreciation [Line Items]
Generation	9,199,012	8,916,872
Transmission	3,290,477	3,095,907
Distribution	7,107,007	6,690,697
General plant	1,241,389	1,132,816
Plant in service and held for future use	20,837,885	19,836,292
Accumulated depreciation and amortization	(7,110,310)	(6,637,857)
Net	13,727,575	13,198,435
Construction work in progress	937,384	808,133
Intangible assets, net of accumulated amortization	282,570	290,564
Nuclear fuel, net of accumulated amortization	113,645	123,500
Total property, plant and equipment	15,159,210	14,522,538
Electric Service | Variable Interest Entity
Utility Plant and Depreciation [Line Items]
Total property, plant and equipment	$ 98,036	$ 101,906