Utility Rate Cases (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended						1 Months Ended	12 Months Ended			1 Months Ended			0 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended	9 Months Ended	12 Months Ended		1 Months Ended		12 Months Ended	1 Months Ended			1 Months Ended				12 Months Ended	1 Months Ended									12 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 IPL [Member]	Dec. 31, 2010 IPL [Member]	Dec. 31, 2009 IPL [Member]	Jan. 31, 2011 WPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Dec. 31, 2009 WPL [Member]	Jan. 31, 2010 2008 Test Year Retail Electric Request IA [Member] IPL [Member]	Mar. 31, 2009 2008 Test Year Retail Electric Request IA [Member] IPL [Member]	Aug. 31, 2011 2009 Test Year Retail Electric MN Final Order [Member] IPL [Member]	Jul. 06, 2010 2009 Test Year Retail Electric Request MN [Member] IPL [Member]	May 31, 2010 2009 Test Year Retail Electric Request MN [Member] IPL [Member]	Dec. 31, 2011 2009 Test Year Retail Electric Request MN [Member] Alliant Energy And IPL [Member]	Dec. 31, 2010 2009 Test Year Retail Electric Request MN [Member] Alliant Energy And IPL [Member]	Mar. 31, 2010 2009 Test Year Retail Electric Request IA [Member] IPL [Member]	Sep. 30, 2011 2009 Test Year Retail Electric Request IA [Member] IPL [Member]	Dec. 31, 2011 2009 Test Year Retail Electric Request IA [Member] Alliant Energy And IPL [Member]	Dec. 31, 2010 2009 Test Year Retail Electric Request IA [Member] Alliant Energy And IPL [Member]	Feb. 28, 2011 2009 Test Year Retail Electric Request IA Final Order [Member] IPL [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]	Dec. 31, 2010 2010 Test Year Retail Electric Fuel Related Request [Member] WPL [Member]	Apr. 30, 2010 2010 Test Year Retail Electric Fuel Related Request [Member] WPL [Member]	Dec. 31, 2011 2010 Test Year Retail Electric Fuel Related Request [Member] Alliant Energy And WPL [Member]	Jan. 31, 2011 2011 Test Year Retail Electric Request Final Order [Member]	Dec. 31, 2010 2011 Test Year Retail Electric Request Final Order [Member] WPL [Member]	Aug. 31, 2011 2012 Test Year Retail Electric Fuel Related Request [Member] WPL [Member]	May 31, 2011 2012 Test Year Retail Electric Fuel Related Request [Member] WPL [Member]	Dec. 31, 2012 2012 Test Year Retail Electric Fuel Related Request Final Order [Member]	Dec. 31, 2011 2012 Test Year Retail Electric Fuel Related Request Final Order [Member] WPL [Member]	Aug. 31, 2011 Higher Base Rates [Member] IPL [Member]	Aug. 31, 2011 Renewable Energy Rider [Member] IPL [Member]	Aug. 31, 2011 IPL Electric Transmission Assets Sale [Member] 2009 Test Year Retail Electric MN Final Order [Member]	Dec. 31, 2009 Retail Electric Rate [Member] 2010 Test Year Retail Rate Case [Member] WPL [Member]	Dec. 31, 2009 Retail Natural Gas Rate [Member] 2010 Test Year Retail Rate Case [Member] WPL [Member]	Jan. 31, 2010 Flood Related [Member] 2008 Test Year Retail Electric Request IA [Member] IPL [Member]	Jan. 31, 2011 Flood Related [Member] 2009 Test Year Retail Electric Request IA Final Order [Member] IPL [Member]	Jan. 31, 2010 Sutherland Four [Member] 2008 Test Year Retail Electric Request IA [Member] IPL [Member]	Dec. 31, 2011 Sutherland Four [Member] 2009 Test Year Retail Electric Request MN [Member] IPL [Member]	Jan. 31, 2011 Whispering Willow-East Wind Project [Member] 2009 Test Year Retail Electric Request IA [Member] IPL [Member]	Dec. 31, 2011 MN [Member] Retail Electric Rate [Member] IPL [Member]	Dec. 31, 2010 MN [Member] Retail Electric Rate [Member] IPL [Member]	Dec. 31, 2011 Cross State Air Pollution Rule [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member]
Utility Rate Cases [Line Items]
Reduction in interim rates for its customers, amount																								$ 5
Increase in interim rates for its customers, amount												84		14				119					9	5	9									8	2	1
Increase in annual rates for its customers, amount													11		15							114				9		8	8	31	13		4				59	6
Electric	2,635.8	2,674.2	2,475.9	1,408.3	1,464.3	1,315.6		1,227.5	1,209.9	1,160.3						12.0	5.0			122.0	96.0
Increase in annual rates for its customers															22.00%							10.00%							1.00%								6.00%	2.00%
Refund to customers																			5																									2	1
Refund reserve																4											1
Recovery of previously incurred costs																																							8.0	7.0	26.0	2.0	26.0
Decrease in annual electric retail rates							1
Deferred compliance costs tolerance band																																														0.00%
Rate of fuel costs capacity																																2.00%
Transmission costs expected to be billed											$ 46