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Summary of Significant Accounting Policies - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2024
USD ($)
segment
plan
foundation
plant
reportingUnit
Dec. 31, 2023
USD ($)
foundation
Dec. 31, 2022
USD ($)
foundation
Property, Plant and Equipment [Line Items]      
Number of reportable segments | segment 3    
Number of jointly owned electric generation plants | plant 2    
Number of reporting units | reportingUnit 3    
Amortization period 10 years    
Amortization expense $ 3.0 $ 1.3 $ 1.4
Number of plans offered | plan 2    
Deferred compensation liability, company contributions $ 29.1 24.6  
Deferred compensation arrangement, contributions by employer 1.3 1.2 0.9
Gains or (losses) recognized due to changes in payment obligations (3.3) $ (3.3) $ 3.1
Coyote Creek Mining Company, L.L.C. (CCMC) | Lignite Sales Agreement | OTP      
Property, Plant and Equipment [Line Items]      
Variable interest entity, reporting entity involvement, maximum loss exposure, amount $ 40.0    
Variable interest entity, reporting entity involvement, maximum loss exposure (as a percent) 35.00%    
Contribution Obligation | Subsidiaries      
Property, Plant and Equipment [Line Items]      
Number of foundations transacting with | foundation 2 2  
Contribution obligations $ 5.5 $ 5.5  
Contribution Obligation Paid | Subsidiaries      
Property, Plant and Equipment [Line Items]      
Number of foundations transacting with | foundation 2 2 2
Amounts of transaction $ 5.5 $ 4.3 $ 4.5
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 $ 1.9 $ 1.9 $ 0.9