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Select Balance Sheet Information - Schedule of Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Sep. 30, 2023
Dec. 31, 2022
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, net of accumulated depreciation $ 2,387,260 $ 2,212,717
Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,138,841 2,958,311
Less Accumulated Depreciation and Amortization 889,136 859,988
Property, Plant and Equipment, net of accumulated depreciation 2,249,705 2,098,323
Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 343,526 309,098
Less Accumulated Depreciation and Amortization 205,971 194,704
Property, Plant and Equipment, net of accumulated depreciation 137,555 114,394
Electric Plant in Service | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,009,031 2,844,379
Construction Work in Progress | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 129,810 113,932
Construction Work in Progress | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 39,885 15,170
Nonelectric Property, Plant and Equipment in Service | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 303,641 $ 293,928