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LONG-TERM DEBT (Details) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended
May 31, 2025
Apr. 30, 2025
Mar. 31, 2025
Mar. 31, 2024
Debt Instrument [Line Items]        
Interest expense     $ 223.0 $ 192.0
5.20% MERC Senior Notes due 5/1/2030 | MERC | Subsequent event        
Debt Instrument [Line Items]        
Proceeds from issuance of debt   $ 50.0    
Interest rate on long-term debt   5.20%    
5.20% MGU Senior Notes due 5/1/2030 | MGU | Subsequent event        
Debt Instrument [Line Items]        
Proceeds from issuance of debt   $ 75.0    
Interest rate on long-term debt   5.20%    
2.69% MGU Senior Notes due 5/1/2025 | MGU | Subsequent event        
Debt Instrument [Line Items]        
Interest rate on long-term debt 2.69%      
Repayment of long-term debt $ 60.0      
2.69% MERC Senior Notes due 5/1/2025 | MERC | Subsequent event        
Debt Instrument [Line Items]        
Interest rate on long-term debt 2.69%      
Repayment of long-term debt $ 50.0      
WEC Energy Group | WEC 4.375% Convertible Notes due June 1, 2027        
Debt Instrument [Line Items]        
Senior Notes     862.5  
Unamortized debt issuance costs     (7.2)  
Net carrying amount     855.3  
Fair value amount     1,009.0  
Contractual interest expense     9.4  
Amortization of debt issuance costs     0.8  
Interest expense     10.2  
WEC Energy Group | WEC 4.375% Convertible Notes due June 1, 2029        
Debt Instrument [Line Items]        
Senior Notes     862.5  
Unamortized debt issuance costs     (8.3)  
Net carrying amount     854.2  
Fair value amount     1,029.2  
Contractual interest expense     9.4  
Amortization of debt issuance costs     0.5  
Interest expense     $ 9.9