Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013 IPL [Member]	Dec. 31, 2012 IPL [Member]	Dec. 31, 2012 WPL [Member]	Mar. 31, 2013 Commodity Contracts [Member]	Dec. 31, 2012 Commodity Contracts [Member]	Mar. 31, 2012 Commodity Contracts [Member]	Dec. 31, 2011 Commodity Contracts [Member]	Mar. 31, 2013 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2012 Commodity Contracts [Member] IPL [Member]	Mar. 31, 2012 Commodity Contracts [Member] IPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] IPL [Member]	Mar. 31, 2013 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2012 Commodity Contracts [Member] WPL [Member]	Mar. 31, 2012 Commodity Contracts [Member] WPL [Member]	Dec. 31, 2011 Commodity Contracts [Member] WPL [Member]	Mar. 31, 2013 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member]	Mar. 31, 2013 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] IPL [Member]	Mar. 31, 2013 Excluding Financial Transmission Rights [Member] Commodity Contracts [Member] WPL [Member]	Mar. 31, 2013 Financial Transmission Rights [Member]	Mar. 31, 2013 Financial Transmission Rights [Member] IPL [Member]	Mar. 31, 2013 Financial Transmission Rights [Member] WPL [Member]
Cumulative preferred stock rate	5.10%	8.375%	4.50%
Fair value, net derivative liabilities																$ 1.4	$ 0.3	$ 1.1
Fair value, net derivative assets				$ 4.9	$ 11.9	$ (15.1)	$ (0.9)	$ 4.3	$ 12.5	$ (11.4)	$ 4.3	$ 0.6	$ (0.6)	$ (3.7)	$ (5.2)				$ 6.3	$ 4.6	$ 1.7