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Regulatory Matters (Narrative) (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2020
Dec. 31, 2018
Regulatory Matters [Line Items]        
Regulatory assets   $ 1,840.0   $ 1,737.3
Regulatory liabilities   1,451.0   1,493.2
IPL [Member]        
Regulatory Matters [Line Items]        
Regulatory assets   1,404.0   1,279.0
Regulatory liabilities   733.6   $ 754.9
2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Interim rate annual increase, amount   90.0    
Forecast [Member] | 2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Requested rate annual increase, amount     $ 127.0  
Billing credit, period     12 months  
Forecast [Member] | 2020 Forward-looking Test Period Retail Gas [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Requested rate annual increase, amount     $ 12.0  
Tax-related [Member] | 2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Regulatory liabilities   27.0    
Revenue Subject to Refund [Member] | 2020 Forward-looking Test Period Retail Electric [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Regulatory liabilities   8.0    
Analog Electric Meters [Member] | Alliant Energy And IPL [Member]        
Regulatory Matters [Line Items]        
Regulatory assets   $ 36.0    
DAEC PPA [Member]        
Regulatory Matters [Line Items]        
PPA term amendment   5 years    
Recovery period, authorized   5 years    
DAEC PPA [Member] | Forecast [Member]        
Regulatory Matters [Line Items]        
Buyout payment $ 110.0