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Common Equity (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
1 Months Ended 12 Months Ended
Jul. 31, 2008
Dec. 31, 2011
Shares available for issuance, savings plan and shareowner direct plan   9.1
Ownership percentage   15.00%
Exercise price, rights   110.00
IPL [Member]
   
Common equity ratio   42.00%
Retained earnings free of dividend restrictions   $ 351
FERC order to pay common equity distributions, maximum 400  
WPL [Member]
   
Common equity ratio   51.01%
Dividends prohibited   112
Retained earnings free of dividend restrictions   $ 112
Minimum [Member]
   
Ownership percentage   10.00%
Minimum [Member] | WPL [Member]
   
Common equity ratio   25.00%