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Fair Value Measurements (Narrative) (Details)
1 Months Ended9 Months Ended12 Months Ended9 Months Ended12 Months Ended
Apr. 30, 2011
Sep. 30, 2011
IPL [Member]
Dec. 31, 2010
IPL [Member]
Sep. 30, 2011
WPL [Member]
Dec. 31, 2010
WPL [Member]
Cumulative preferred stock rate7.10%8.375%7.10%4.50%4.50%