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Select Balance Sheet Information - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, net of accumulated depreciation	$ 2,083,223	$ 2,049,273
Electric Plant in Service
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,806,273	2,734,430
Less Accumulated Depreciation and Amortization	822,441	778,988
Property, Plant and Equipment, net of accumulated depreciation	1,983,832	1,955,442
Nonelectric Property, Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	279,444	268,020
Less Accumulated Depreciation and Amortization	180,053	174,189
Property, Plant and Equipment, net of accumulated depreciation	99,391	93,831
Electric Plant in Service | Electric Plant in Service
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,705,778	2,531,352
Construction Work in Progress | Electric Plant in Service
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	100,495	203,078
Construction Work in Progress | Nonelectric Property, Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	15,564	9,290
Nonelectric Property, Plant and Equipment in Service | Nonelectric Property, Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 263,880	$ 258,730

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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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