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REGULATORY ENVIRONMENT - COVID-19 (Details)
$ in Millions
Jun. 18, 2020
USD ($)
Mar. 24, 2020
order
Jun. 30, 2020
USD ($)
Dec. 31, 2019
USD ($)
Public Utilities, General Disclosures [Line Items]        
Regulatory assets     $ 3,524.5 $ 3,527.6
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      
Illinois Commerce Commission (ICC) | COVID-19        
Public Utilities, General Disclosures [Line Items]        
Recovery period of Special Purpose Rider 36 months      
Regulatory assets     $ 8.5  
Illinois Commerce Commission (ICC) | PGL        
Public Utilities, General Disclosures [Line Items]        
Amount available through bill payment assistance program $ 12.0      
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