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GOODWILL AND INTANGIBLES - GOODWILL (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
USD ($)
Sep. 30, 2026
USD ($)
Goodwill balance by segment    
Changes to the carrying amount of goodwill   $ 0.0
Goodwill $ 3,052.8 3,052.8
Accumulated impairment losses 0.0 0.0
Goodwill impairment loss 0.0  
Wisconsin    
Goodwill balance by segment    
Goodwill 2,104.3 2,104.3
Illinois    
Goodwill balance by segment    
Goodwill 758.7 758.7
Other States    
Goodwill balance by segment    
Goodwill 183.2 183.2
Non-Utility Energy Infrastructure    
Goodwill balance by segment    
Goodwill $ 6.6 $ 6.6