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Summary of Significant Accounting Policies - Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
USD ($)
segment
foundation
plant
reportingUnit
plan
line
Dec. 31, 2022
USD ($)
foundation
Dec. 31, 2021
USD ($)
foundation
Property, Plant and Equipment [Line Items]      
Number of operating segments | segment 3    
Investments gain (loss) $ 7,222 $ (3,296) $ 4,524
Allowance for equity funds used during construction   1,700 800
Number of jointly owned electric generation plants | plant 2    
Number of jointly owned major transmission lines | line 5    
Number of reporting units | reportingUnit 3    
Amortization period 10 years    
Amortization expense $ 1,300 1,400 500
Number of plans offered | plan 2    
Deferred compensation liability, company contributions $ 24,600 20,600  
Deferred compensation arrangement, contributions by employer 1,200 900 1,100
Deferred compensation liability, gain (loss) due to payment obligations (3,300) 3,100 $ (2,200)
Contribution Obligation | Related Party      
Property, Plant and Equipment [Line Items]      
Related party transaction, amounts of transaction $ 5,500 $ 4,300  
Number of foundations transacting with | foundation 2 2 2
Contribution Obligation Paid | Related Party      
Property, Plant and Equipment [Line Items]      
Related party transaction, amounts of transaction $ 4,300 $ 4,500 $ 3,800
Number of foundations transacting with | foundation 2 2 2
OTP | Coyote Creek Mining Company, L.L.C. (CCMC) | Lignite Sales Agreement      
Property, Plant and Equipment [Line Items]      
Variable interest entity, reporting entity involvement, maximum loss exposure, amount $ 40,000    
Variable interest entity, reporting entity involvement, maximum loss exposure (as a percent) 35.00%    
Minimum      
Property, Plant and Equipment [Line Items]      
Intangible asset, useful life 15 years    
Operating lease, term 1 year    
Maximum      
Property, Plant and Equipment [Line Items]      
Intangible asset, useful life 20 years    
Operating lease, term 10 years    
Electric Plant in Service      
Property, Plant and Equipment [Line Items]      
Interest costs capitalized $ 1,900 $ 900 $ 600