Utility Rate Cases (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		1 Months Ended		0 Months Ended	1 Months Ended	9 Months Ended			1 Months Ended	9 Months Ended			1 Months Ended			9 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended							3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Aug. 31, 2011 IPL Electric Transmission Assets Sale [Member] 2009 Test Year Retail Electric MN Final Order [Member]	Aug. 31, 2011 IPL [Member] 2009 Test Year Retail Electric MN Final Order [Member]	Jul. 06, 2010 IPL [Member] 2009 Test Year Retail Electric Request MN [Member]	May 31, 2010 IPL [Member] 2009 Test Year Retail Electric Request MN [Member]	Sep. 30, 2011 IPL [Member] 2009 Test Year Retail Electric Request MN [Member]	Sep. 30, 2011 Alliant Energy And IPL [Member] 2009 Test Year Retail Electric Request MN [Member]	Sep. 30, 2010 Alliant Energy And IPL [Member] 2009 Test Year Retail Electric Request MN [Member]	Mar. 31, 2010 IPL [Member] 2009 Test Year Retail Electric Request IA [Member]	Sep. 30, 2011 IPL [Member] 2009 Test Year Retail Electric Request IA [Member]	Sep. 30, 2011 Alliant Energy And IPL [Member] 2009 Test Year Retail Electric Request IA [Member]	Sep. 30, 2010 Alliant Energy And IPL [Member] 2009 Test Year Retail Electric Request IA [Member]	Feb. 28, 2011 IPL [Member] 2009 Test Year Retail Electric Request IA Final Order [Member]	Dec. 31, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member]	Apr. 30, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member]	Sep. 30, 2011 Alliant Energy And WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member]	Jun. 30, 2010 2010 Test Year Retail Electric Fuel Related Request [Member]	Dec. 31, 2010 2010 Test Year Retail Electric Fuel Related Request [Member]	Jan. 31, 2011 WPL [Member] 2011 Test Year Retail Electric Request Final Order [Member]	Dec. 31, 2010 WPL [Member] 2011 Test Year Retail Electric Request Final Order [Member]	Jan. 31, 2011 2011 Test Year Retail Electric Request Final Order [Member]	Aug. 31, 2011 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request [Member]	May 31, 2011 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request [Member]	Aug. 31, 2011 IPL [Member] Higher Base Rates [Member]	Aug. 31, 2011 IPL [Member] Renewable Energy Rider [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2010 IPL [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2010 IPL [Member]	Jan. 31, 2011 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]
Reduction in interim rates for its customers, amount																					$ 5
Increase in interim rates for its customers, amount					1		14					119					9			9	5						8	2
Increase in annual rates for its customers, amount						11		15								114		9				8		8	20	13
Electric revenue	796.9	842.0	2,037.7	2,074.2						9.0	4.0			95.0	72.0														443.2	492.4	1,097.3	1,146.8		353.7	349.6	940.4	927.4
Increase in annual rates for its customers								22.00%								10.00%							1.00%
Refund to customers										3			5						2
Recovery of previously incurred costs									2
Decrease in annual electric retail rates																																	$ 1