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Common Equity (Narrative) (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Apr. 20, 2016
shares
Sep. 30, 2016
USD ($)
shares
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
shares
Sep. 30, 2015
USD ($)
shares
Dec. 31, 2015
shares
Common Equity [Line Items]            
Proceeds from issuance of common stock, net       $ 20.4 $ 145.4  
Common stock split conversion ratio (2:1) 2          
Common stock, shares authorized (in shares) | shares 240,000,000 480,000,000 [1]   480,000,000 [1]   480,000,000 [1]
Other comprehensive income   $ 0.0 $ 0.0 $ 0.0 0.0  
IPL [Member]            
Common Equity [Line Items]            
Common stock, shares authorized (in shares) | shares   24,000,000   24,000,000   24,000,000
Retained earnings free of dividend restrictions   $ 632.0   $ 632.0    
Restricted net assets of subsidiaries   1,500.0   1,500.0    
Capital contributions from parent       65.0 100.0  
Common stock dividends to parent       114.0    
Other comprehensive income   $ 0.0 0.0 $ 0.0 0.0  
WPL [Member]            
Common Equity [Line Items]            
Common stock, shares authorized (in shares) | shares   18,000,000   18,000,000   18,000,000
Retained earnings free of dividend restrictions   $ 34.0   $ 34.0    
Restricted net assets of subsidiaries   1,800.0   1,800.0    
Common stock dividends to parent       101.2    
Other comprehensive income   $ 0.0 $ 0.0 $ 0.0 $ 0.0  
At-the-market Offering Program [Member]            
Common Equity [Line Items]            
Common stock issued during the period, At-the-market offering program (in shares) | shares         4,373,234  
Proceeds from issuance of common stock, net         $ 133.0  
Fees and commissions from issuance of common stock         $ 2.0  
[1] Share and per share amounts reflect the effects of a two-for-one common stock split distributed in May 2016.