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Derivative Instruments Derivative Instruments (Credit Risk-related Contingent Features) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Derivative [Line Items]    
Aggregate fair value $ 9.0 $ 20.8
Credit support to be posted if triggered 9.0 20.8
IPL [Member]
   
Derivative [Line Items]    
Aggregate fair value 2.2 5.2
Credit support to be posted if triggered 2.2 5.2
WPL [Member]
   
Derivative [Line Items]    
Aggregate fair value 6.8 15.6
Credit support to be posted if triggered $ 6.8 $ 15.6