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Benefit Plans (Amounts Recognized On The Consolidated Balance Sheets) (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Amounts recognized on the balance sheets consist of:    
Pension and other benefit obligations $ (484.0) $ (509.1)
Pension Plans, Defined Benefit [Member]    
Amounts recognized on the balance sheets consist of:    
Non-current assets 0.0 0.0
Current liabilities (14.3) (2.3)
Pension and other benefit obligations (335.0) (364.1)
Net amounts recognized at December 31 (349.3) (366.4)
Amounts recognized in Regulatory Assets consist of:    
Net actuarial loss 485.3 505.2
Prior service credit (5.1) (5.8)
Net amount recognized at December 31 480.2 499.4
Other Postretirement Benefits Plans [Member]    
Amounts recognized on the balance sheets consist of:    
Non-current assets 9.8 7.5
Current liabilities (8.1) (9.6)
Pension and other benefit obligations (110.4) (104.9)
Net amounts recognized at December 31 (108.7) (107.0)
Amounts recognized in Regulatory Assets consist of:    
Net actuarial loss 45.2 44.5
Prior service credit (0.9) (1.1)
Net amount recognized at December 31 44.3 43.4
IPL [Member]    
Amounts recognized on the balance sheets consist of:    
Pension and other benefit obligations (167.7) (178.4)
IPL [Member] | Pension Plans, Defined Benefit [Member]    
Amounts recognized on the balance sheets consist of:    
Non-current assets 0.0 0.0
Current liabilities (0.5) (0.6)
Pension and other benefit obligations (144.7) (155.5)
Net amounts recognized at December 31 (145.2) (156.1)
Amounts recognized in Regulatory Assets consist of:    
Net actuarial loss 208.2 218.8
Prior service credit (1.7) (1.8)
Net amount recognized at December 31 206.5 217.0
IPL [Member] | Other Postretirement Benefits Plans [Member]    
Amounts recognized on the balance sheets consist of:    
Non-current assets 6.6 4.7
Current liabilities (1.8) (1.9)
Pension and other benefit obligations (18.6) (18.6)
Net amounts recognized at December 31 (13.8) (15.8)
Amounts recognized in Regulatory Assets consist of:    
Net actuarial loss 16.6 18.8
Prior service credit 0.0 0.0
Net amount recognized at December 31 16.6 18.8
WPL [Member]    
Amounts recognized on the balance sheets consist of:    
Pension and other benefit obligations (210.8) (217.7)
WPL [Member] | Pension Plans, Defined Benefit [Member]    
Amounts recognized on the balance sheets consist of:    
Non-current assets 0.0 0.0
Current liabilities (0.1) (0.1)
Pension and other benefit obligations (144.0) (154.3)
Net amounts recognized at December 31 (144.1) (154.4)
Amounts recognized in Regulatory Assets consist of:    
Net actuarial loss 212.0 223.0
Prior service credit (1.2) (1.3)
Net amount recognized at December 31 210.8 221.7
WPL [Member] | Other Postretirement Benefits Plans [Member]    
Amounts recognized on the balance sheets consist of:    
Non-current assets 3.2 2.8
Current liabilities (6.0) (7.4)
Pension and other benefit obligations (65.4) (61.9)
Net amounts recognized at December 31 (68.2) (66.5)
Amounts recognized in Regulatory Assets consist of:    
Net actuarial loss 22.2 19.9
Prior service credit (0.9) (1.1)
Net amount recognized at December 31 $ 21.3 $ 18.8