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REGULATORY ENVIRONMENT - COVID-19 (Details)
$ in Millions
1 Months Ended
Mar. 31, 2021
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
order
Sep. 30, 2021
USD ($)
Apr. 30, 2021
USD ($)
Dec. 31, 2020
USD ($)
Public Utilities, General Disclosures [Line Items]            
Regulatory assets       $ 3,628.1   $ 3,544.1
Public Service Commission of Wisconsin (PSCW)            
Public Utilities, General Disclosures [Line Items]            
Number of orders issued in response to COVID-19 | order     2      
Illinois Commerce Commission (ICC)            
Public Utilities, General Disclosures [Line Items]            
Additional period of time deposit requirements will be waived   4 months        
Percentage below federal poverty level for which customer disconnection is disallowed 300.00%          
Illinois Commerce Commission (ICC) | COVID-19            
Public Utilities, General Disclosures [Line Items]            
Recovery period of regulatory asset   36 months        
Regulatory assets       $ 25.3    
Illinois Commerce Commission (ICC) | PGL            
Public Utilities, General Disclosures [Line Items]            
Amount available through bill payment assistance program   $ 12.0        
Additional amount available through bill payment assistance program $ 6.0          
Amount of funds provided through bill payment assistance program         $ 18.0  
Illinois Commerce Commission (ICC) | NSG            
Public Utilities, General Disclosures [Line Items]            
Amount available through bill payment assistance program   $ 1.2        
Minnesota Public Utilities Commission (MPUC)            
Public Utilities, General Disclosures [Line Items]            
Period of time after peacetime emergency that order will be effective 90 days