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Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Current derivative assets $ 35.9 $ 29.4
Non-current derivative assets 2.0 12.0
Current derivative liabilities 17.8 13.3
Non-current derivative liabilities 27.6 15.3
IPL [Member]    
Derivatives, Fair Value [Line Items]    
Current derivative assets 28.7 19.1
Non-current derivative assets 0.6 1.7
Current derivative liabilities 4.1 2.7
Non-current derivative liabilities 12.2 5.6
WPL [Member]    
Derivatives, Fair Value [Line Items]    
Current derivative assets 7.2 10.3
Non-current derivative assets 1.4 10.3
Current derivative liabilities 13.7 10.6
Non-current derivative liabilities $ 15.4 $ 9.7