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Benefit Plans (Defined Contribution Retirement Plans) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
401(k) costs $ 5.6 $ 6.1 $ 12.3 $ 12.0
IPL [Member]        
401(k) costs [1] 2.9 2.6 6.3 5.7
WPL [Member]        
401(k) costs [1] $ 2.6 $ 3.3 $ 5.5 $ 5.9
[1] IPL’s and WPL’s amounts include allocated costs associated with Corporate Services employees.