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REGULATORY ENVIRONMENT - COVID-19 (Details)
$ in Millions
1 Months Ended
Mar. 18, 2021
USD ($)
Aug. 01, 2020
Mar. 31, 2021
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
order
Apr. 30, 2021
USD ($)
Dec. 31, 2020
USD ($)
Public Utilities, General Disclosures [Line Items]              
Regulatory assets     $ 3,793.8       $ 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     $ 20.0        
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            
Illinois Commerce Commission (ICC) | PGL | Subsequent event              
Public Utilities, General Disclosures [Line Items]              
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   60 days 90 days