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Select Balance Sheet Information - Schedule of Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, net of accumulated depreciation $ 2,820,689 $ 2,692,460
Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,569,042 3,412,833
Less Accumulated Depreciation and Amortization 921,038 899,049
Property, Plant and Equipment, net of accumulated depreciation 2,648,004 2,513,784
Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 413,416 403,101
Less Accumulated Depreciation and Amortization 240,731 224,425
Property, Plant and Equipment, net of accumulated depreciation 172,685 178,676
Electric Plant in Service | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,234,585 3,180,943
Construction Work in Progress | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 334,457 231,890
Construction Work in Progress | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 12,223 40,536
Nonelectric Property, Plant and Equipment in Service | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 401,193 $ 362,565