Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 IPL [Member]	Sep. 30, 2012 WPL [Member]	Sep. 30, 2012 Commodity Contracts [Member]	Jun. 30, 2012 Commodity Contracts [Member]	Dec. 31, 2011 Commodity Contracts [Member]	Sep. 30, 2011 Commodity Contracts [Member]	Jun. 30, 2011 Commodity Contracts [Member]	Dec. 31, 2010 Commodity Contracts [Member]	Sep. 30, 2012 Commodity Contracts [Member] IPL [Member]	Jun. 30, 2012 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] IPL [Member]	Sep. 30, 2011 Commodity Contracts [Member] IPL [Member]	Jun. 30, 2011 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] IPL [Member]	Sep. 30, 2012 Commodity Contracts [Member] WPL [Member]	Jun. 30, 2012 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] WPL [Member]	Sep. 30, 2011 Commodity Contracts [Member] WPL [Member]	Jun. 30, 2011 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] WPL [Member]	Sep. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] Alliant Energy [Member]	Sep. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] IPL [Member]	Sep. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] WPL [Member]	Sep. 30, 2012 Financial Transmission Rights [Member] Alliant Energy [Member]	Sep. 30, 2012 Financial Transmission Rights [Member] IPL [Member]	Sep. 30, 2012 Financial Transmission Rights [Member] WPL [Member]
Cumulative preferred stock rate	8.375%	4.50%
Fair value			$ 17.6	$ 18.8	$ (0.9)	$ 11.9	$ 18.1	$ 2.8	$ 15.5	$ 14.1	$ 4.3	$ 12.4	$ 17.5	$ 4.3	$ 2.1	$ 4.7	$ (5.2)	$ (0.5)	$ 0.6	$ (1.5)	$ 5.6	$ 1.2	$ 4.4	$ 23.2	$ 16.7	$ 6.5