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Select Balance Sheet Information - Schedule of Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Mar. 31, 2024
Dec. 31, 2023
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, net of accumulated depreciation $ 2,459,189 $ 2,418,375
Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,174,353 3,127,093
Less Accumulated Depreciation and Amortization 864,826 851,148
Property, Plant and Equipment, net of accumulated depreciation 2,309,527 2,275,945
Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 361,149 349,986
Less Accumulated Depreciation and Amortization 211,487 207,556
Property, Plant and Equipment, net of accumulated depreciation 149,662 142,430
Electric Plant in Service | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,030,848 2,989,881
Construction Work in Progress | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 143,505 137,212
Construction Work in Progress | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 38,237 38,062
Nonelectric Property, Plant and Equipment in Service | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 322,912 $ 311,924