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Derivative Instruments (Fair Value Of Financial Instruments) (Details) - Commodity Contracts [Member] - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Current derivative assets $ 26.7 $ 29.4
Non-current derivative assets 2.7 12.0
Current derivative liabilities 18.5 13.3
Non-current derivative liabilities 26.6 15.3
IPL [Member]    
Derivatives, Fair Value [Line Items]    
Current derivative assets 20.3 19.1
Non-current derivative assets 0.8 1.7
Current derivative liabilities 4.6 2.7
Non-current derivative liabilities 14.1 5.6
WPL [Member]    
Derivatives, Fair Value [Line Items]    
Current derivative assets 6.4 10.3
Non-current derivative assets 1.9 10.3
Current derivative liabilities 13.9 10.6
Non-current derivative liabilities $ 12.5 $ 9.7