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Regulatory Matters (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Regulatory Matters [Line Items]        
Regulatory assets $ 1,963.0     $ 1,915.1
Regulatory liabilities 696.6     681.0
IPL [Member]        
Regulatory Matters [Line Items]        
Regulatory assets 1,500.8     1,458.8
Regulatory liabilities 448.3     430.8
WPL [Member]        
Regulatory Matters [Line Items]        
Regulatory assets 462.2     456.3
Regulatory liabilities 248.3     250.2
2017 and 2018 Test Period Retail Electric [Member] | Alliant Energy and WPL [Member]        
Regulatory Matters [Line Items]        
Increase in electric revenues 22.0      
2017 and 2018 Test Period Retail Gas [Member] | Alliant Energy and WPL [Member]        
Regulatory Matters [Line Items]        
Increase in gas revenues $ 2.0      
2017 Test Year Retail Electric Fuel Related [Member] | WPL [Member]        
Regulatory Matters [Line Items]        
Annual bandwidth for fuel-related costs 2.00%      
2016 Test Year Retail Electric Fuel-Related [Member] | WPL [Member]        
Regulatory Matters [Line Items]        
Annual bandwidth for fuel-related costs 2.00%      
Scenario, Forecast [Member] | 2016 Test Year Retail Electric [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Requested rate increase (decrease), amount   $ 176.0    
Requested rate increase (decrease), percent   12.00%    
Interim rate increase (decrease), amount     $ 102.0  
Interim rate increase (decrease), percent     7.00%  
Scenario, Forecast [Member] | 2017 and 2018 Test Period Retail Electric [Member] | WPL [Member]        
Regulatory Matters [Line Items]        
Authorized increase (decrease) in final rates, amount     $ 9.0  
Authorized increase (decrease) in final rates, percent     1.00%  
Increase in base rates     $ 60.0  
Reduction in fuel-related costs     51.0  
Scenario, Forecast [Member] | 2017 and 2018 Test Period Retail Gas [Member] | WPL [Member]        
Regulatory Matters [Line Items]        
Authorized increase (decrease) in final rates, amount     $ 9.0  
Authorized increase (decrease) in final rates, percent     13.00%  
Scenario, Forecast [Member] | 2017 Test Year Retail Electric Fuel Related [Member] | WPL [Member]        
Regulatory Matters [Line Items]        
Under-collection of retail fuel-related costs     $ 8.0  
FERC [Member] | MISO Transmission Owner Complaints [Member]        
Regulatory Matters [Line Items]        
Base return on equity for refund period from November 12, 2013 through February 11, 2015, percent 10.32%      
Refund received subject to final true-up $ 51.0      
FERC [Member] | MISO Transmission Owner Complaints [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Refund received subject to final true-up 40.0      
FERC [Member] | MISO Transmission Owner Complaints [Member] | WPL [Member]        
Regulatory Matters [Line Items]        
Refund received subject to final true-up 11.0      
Commodity cost recovery [Member] | 2017 Test Year Retail Electric Fuel Related [Member] | Alliant Energy and WPL [Member]        
Regulatory Matters [Line Items]        
Regulatory assets 1.0      
Commodity cost recovery [Member]        
Regulatory Matters [Line Items]        
Regulatory liabilities 32.9     30.8
Commodity cost recovery [Member] | IPL [Member]        
Regulatory Matters [Line Items]        
Regulatory liabilities 20.1     17.8
Commodity cost recovery [Member] | WPL [Member]        
Regulatory Matters [Line Items]        
Regulatory liabilities 12.8     $ 13.0
Commodity cost recovery [Member] | 2016 Test Year Retail Electric Fuel-Related [Member] | Alliant Energy and WPL [Member]        
Regulatory Matters [Line Items]        
Regulatory liabilities $ 9.0