Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 IPL [Member]	Dec. 31, 2012 WPL [Member]	Dec. 31, 2012 Commodity Contracts [Member]	Dec. 31, 2011 Commodity Contracts [Member]	Dec. 31, 2010 Commodity Contracts [Member]	Dec. 31, 2012 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2012 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2012 Excluding FTRs [Member] Commodity Contracts [Member]	Dec. 31, 2012 Excluding FTRs [Member] Commodity Contracts [Member] IPL [Member]	Dec. 31, 2012 Excluding FTRs [Member] Commodity Contracts [Member] WPL [Member]	Dec. 31, 2012 FTRs [Member]	Dec. 31, 2012 FTRs [Member] IPL [Member]	Dec. 31, 2012 FTRs [Member] WPL [Member]
Cumulative preferred stock rate	8.375%	4.50%
Fair value			$ 11.9	$ (0.9)	$ 2.8	$ 12.5	$ 4.3	$ 4.3	$ (0.6)	$ (5.2)	$ (1.5)	$ (3.2)	$ 1.5	$ (4.7)	$ 15.1	$ 11.0	$ 4.1