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INCOME TAXES - STATUTORY RATE RECONCILIATION (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Statutory rate reconciliation, amount        
Income before income taxes $ 145.6 $ 118.8 $ 309.6 $ 278.6
United States federal statutory income tax rate, amount 30.6 24.9 65.0 58.5
State and local income taxes net of federal tax effect, amount $ 8.8 $ 7.4 $ 18.7 $ 17.3
Tax Jurisdiction of Domicile [Extensible Enumeration] UNITED STATES UNITED STATES UNITED STATES UNITED STATES
Tax credits, amount        
PTCs, net, amount $ (8.7) $ (5.7) $ (21.0) $ (15.6)
Other, amount (1.2) (1.1) (2.8) (2.8)
Nontaxable or nondeductible items, amount        
AFUDC-Equity, amount (1.7) (1.0) (4.1) (2.0)
Other, amount 0.7 0.7 1.9 1.6
Other adjustments, amount        
Federal excess deferred tax amortization, amount (3.9) (3.2) (8.7) (8.2)
Other, net, amount (0.7) 0.5 (0.3) 2.5
Total income tax expense, amount $ 23.9 $ 22.5 $ 48.7 $ 51.3
Statutory rate reconciliation, percent        
United States federal statutory income tax rate, percent 21.00% 21.00% 21.00% 21.00%
State and local income taxes net of federal tax effect, percent 6.00% 6.20% 6.00% 6.20%
Tax credits, percent        
PTCs, net, percent (5.90%) (4.80%) (6.80%) (5.60%)
Other, percent (0.80%) (0.90%) (0.90%) (1.00%)
Nontaxable or nondeductible items, percent        
AFUDC-Equity, percent (1.20%) (0.80%) (1.30%) (0.70%)
Other, percent 0.50% 0.60% 0.60% 0.60%
Other adjustments, percent        
Federal excess deferred tax amortization, percent (2.60%) (2.70%) (2.80%) (2.90%)
Other, net, percent (0.60%) 0.30% (0.10%) 0.80%
Total income tax expense, percent 16.40% 18.90% 15.70% 18.40%
Rate2018And2019 | Public Service Commission of Wisconsin (PSCW) | Tax Cuts And Jobs Act Of 2017        
Income taxes        
Income statement impact of amortizing deferred tax benefits $ 0.0 $ 0.0 $ 0.0 $ 0.0