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COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details)
$ in Millions
 1 Months Ended 6 Months Ended 12 Months Ended 15 Months Ended
May 31, 2022
micrograms
Mar. 31, 2022
micrograms
Jun. 30, 2021
area
Dec. 31, 2020
USD ($)
micrograms
performance_obligations
Jun. 30, 2020
generating_units
Jun. 30, 2022
USD ($)
States
MW
Dec. 31, 2020
USD ($)
performance_obligations
Mar. 31, 2019
MW
Dec. 31, 2021
USD ($)
Oct. 31, 2021
performance_obligations
Manufactured gas plant remediation                    
Regulatory assets | $           $ 3,278.9     $ 3,367.1  
Environmental remediation costs                    
Manufactured gas plant remediation                    
Regulatory assets | $           $ 621.6     630.9  
Cross State Air Pollution Rule - Good Neighbor Plan | Electric                    
Air quality                    
Number of states the EPA is proposing to update and expand cross state air pollution rules to regulate NOx emissions in | States           26        
Cross State Air Pollution Rule - Good Neighbor Plan | Electric | Maximum                    
Air quality                    
RICE unit megawatts | MW           25        
National Ambient Air Quality Standards | Electric                    
Air quality                    
Number of changes to the 2015 ozone standards | performance_obligations                   0
Number of counties to have boundaries revised | area     13              
Number of nonattainment areas as designated by the EPA | area     6              
Number of revisions necessary to meet the 2012 standard for particulate matter | performance_obligations       0            
Current number of micrograms per cubic meter that particulate matter needs to be below       12            
24-hour standard for fine particulate matter       35            
Lowest limit that will cause non-attainment   10                
National Ambient Air Quality Standards | Electric | Maximum                    
Air quality                    
Majority of CASAC members support this range in the peer review they completed   10                
Minority of CASAC members support this range in the peer review they completed   11                
Upper range of the annual standard the EPA staff supported 12                  
Lower range of the annual standard the EPA staff supported 10                  
National Ambient Air Quality Standards | Electric | Minimum                    
Air quality                    
Majority of CASAC members support this range in the peer review they completed   8                
Minority of CASAC members support this range in the peer review they completed   10                
Upper range of the annual standard the EPA staff supported 10                  
Lower range of the annual standard the EPA staff supported 8                  
Climate Change | Electric                    
Air quality                    
Capacity of coal-fired generation retired, in megawatts | MW               1,800    
Capacity of fossil-fueled generation to be retired by the end of 2026, in megawatts | MW           1,600        
Company goal for percentage of carbon emissions reduction below 2005 levels by the end of 2025           60.00%        
Company goal for percentage of carbon emissions reduction below 2005 levels by the end of 2030           80.00%        
Clean Water Act Cooling Water Intake Structure Rule | Electric                    
Water quality                    
Number of generating units that may be retired | generating_units         4          
Steam Electric Effluent Limitation Guidelines | Electric                    
Water quality                    
Number of new ELG rule requirements that affect our electric utilities | performance_obligations             2      
Capital investment to achieve required discharge limits | $       $ 100.0     $ 100.0      
Manufactured Gas Plant Remediation | Natural gas                    
Manufactured gas plant remediation                    
Reserves for future environmental remediation | $           $ 504.3     532.6  
Manufactured Gas Plant Remediation | Natural gas | Environmental remediation costs                    
Manufactured gas plant remediation                    
Regulatory assets | $           $ 621.6     $ 630.9