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INCOME TAXES - STATUTORY RATE RECONCILIATION (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Statutory rate reconciliation, amount        
Income before income taxes $ 328.0 $ 262.5 $ 1,187.2 $ 1,048.7
United States federal statutory income tax rate, amount 68.5 55.6 248.6 220.4
State and local income taxes net of federal tax effect, amount 18.7 15.2 68.0 63.7
Tax credits, amount        
PTCs, net, amount (38.8) (40.5) (168.5) (160.6)
Other, amount (1.5) (1.3) (5.4) (4.6)
Nontaxable or nondeductible items, amount        
AFUDC-Equity, amount (7.8) (2.7) (26.9) (11.2)
Other, amount 2.2 2.3 8.0 5.7
Changes in unrecognized tax benefits, amount        
Changes in unrecognized tax benefits, amount     0.0 0.1
Other adjustments, amount        
Federal excess deferred tax amortization, amount (6.8) (6.8) (25.3) (25.5)
Other, net, amount (7.5) (2.3) (18.4) (7.8)
Total income tax expense, amount $ 27.0 $ 19.5 $ 80.1 $ 80.2
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 5.70% 5.80% 5.70% 6.10%
Tax credits, percent        
PTCs, net, percent (12.00%) (15.30%) (14.20%) (15.30%)
Other, percent (0.40%) (0.50%) (0.50%) (0.40%)
Nontaxable or nondeductible items, percent        
AFUDC-Equity, percent (2.40%) (1.00%) (2.30%) (1.10%)
Other, percent 0.70% 0.90% 0.70% 0.50%
Changes in unrecognized tax benefits, percent        
Changes in unrecognized tax benefits, percent     0.00% 0.00%
Other adjustments, percent        
Federal excess deferred tax amortization, percent (2.10%) (2.60%) (2.10%) (2.50%)
Other, net, percent (2.30%) (0.90%) (1.60%) (0.70%)
Total income tax expense, percent 8.20% 7.40% 6.70% 7.60%
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