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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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