Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 IPL [Member]	Jun. 30, 2012 WPL [Member]	Jun. 30, 2012 Commodity Contracts [Member]	Mar. 31, 2012 Commodity Contracts [Member]	Dec. 31, 2011 Commodity Contracts [Member]	Jun. 30, 2011 Commodity Contracts [Member]	Mar. 31, 2011 Commodity Contracts [Member]	Dec. 31, 2010 Commodity Contracts [Member]	Jun. 30, 2012 Commodity Contracts [Member] IPL [Member]	Mar. 31, 2012 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] IPL [Member]	Jun. 30, 2011 Commodity Contracts [Member] IPL [Member]	Mar. 31, 2011 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] IPL [Member]	Jun. 30, 2012 Commodity Contracts [Member] WPL [Member]	Mar. 31, 2012 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] WPL [Member]	Jun. 30, 2011 Commodity Contracts [Member] WPL [Member]	Mar. 31, 2011 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2010 Commodity Contracts [Member] WPL [Member]	Jun. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] Alliant Energy [Member]	Jun. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] IPL [Member]	Jun. 30, 2012 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] WPL [Member]	Jun. 30, 2012 Financial Transmission Rights [Member] Alliant Energy [Member]	Jun. 30, 2012 Financial Transmission Rights [Member] IPL [Member]	Jun. 30, 2012 Financial Transmission Rights [Member] WPL [Member]
Cumulative preferred stock rate	8.375%	4.50%
Fair value			$ 18.8	$ (15.1)	$ (0.9)	$ 18.1	$ (1.1)	$ 2.8	$ 14.1	$ (11.4)	$ 4.3	$ 17.5	$ 1.4	$ 4.3	$ 4.7	$ (3.7)	$ (5.2)	$ 0.6	$ (2.5)	$ (1.5)	$ 13.7	$ 9.9	$ 3.8	$ 32.5	$ 24.0	$ 8.5