Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 IPL [Member]	Mar. 31, 2012 WPL [Member]	Mar. 31, 2012 Commodity Contracts [Member]	Dec. 31, 2011 Commodity Contracts [Member]	Mar. 31, 2011 Commodity Contracts [Member]	Dec. 31, 2010 Commodity Contracts [Member]	Mar. 31, 2012 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] IPL [Member]	Mar. 31, 2011 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] IPL [Member]	Mar. 31, 2012 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] WPL [Member]	Mar. 31, 2011 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] WPL [Member]	Mar. 31, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] Alliant Energy Corporation [Member]	Mar. 31, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] IPL [Member]	Mar. 31, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] WPL [Member]	Mar. 31, 2012 Financial Transmission Rights [Member] Alliant Energy Corporation [Member]	Mar. 31, 2012 Financial Transmission Rights [Member] IPL [Member]	Mar. 31, 2012 Financial Transmission Rights [Member] WPL [Member]
Cumulative preferred stock rate	8.375%	4.50%
Fair value			$ (15.1)	$ (0.9)	$ (1.1)	$ 2.8	$ (11.4)	$ 4.3	$ 1.4	$ 4.3	$ (3.7)	$ (5.2)	$ (2.5)	$ (1.5)	$ 18.4	$ 14.3	$ 4.1	$ 3.3	$ 2.9	$ 0.4