Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended			1 Months Ended		9 Months Ended	1 Months Ended		9 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended		9 Months Ended	1 Months Ended							9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011 2012 Test Year Retail Electric Fuel Related, Final Order [Member] Cross State Air Pollution Rule [Member]	Sep. 30, 2012 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2012 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2012 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member]	May 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas, Request [Member]	Jun. 30, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member]	Sep. 30, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member]	Oct. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Oral Decision [Member]	Dec. 31, 2011 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Final Order [Member]	Sep. 30, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related, Final Order [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2011 IPL [Member]	Jun. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member]	May 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member]	Sep. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] Tax Benefit Rider [Member]	Aug. 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Settlement [Member]	Jul. 31, 2010 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member]	May 31, 2010 IPL [Member] 2009 Test Year Retail Electric MN, Request [Member]	Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member]	Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] Higher Base Rates [Member]	Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] Renewable Energy Rider [Member]	Nov. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] IPL Electric Transmission Assets Sale [Member]	Sep. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] Sutherland 4 [Member]	Sep. 30, 2012 Alliant Energy and IPL [Member] 2011 Test Year Retail Gas IA, Request [Member]	Sep. 30, 2012 Alliant Energy and IPL [Member] 2011 Test Year Retail Gas IA, Request [Member]
Utility Rate Cases [Line Items]
Reduction in annual rates for its customers, amount											$ 13	$ 25		$ 29									$ 36
Jointly owned utility plant, proportionate ownership share										25.00%
Increase in annual rates for its customers, amount															4							15		11		15	11	8	2	1
Increase in annual rates for its customers, percentage																						6.00%				22.00%
Increase in interim rates for its customers, amount																					9				14
Increase in interim rates for its customers, percentage																					3.00%
Interim rate increase for its customers, actual amount	46.8	46.4	263.9	342.5		17.2	18.9	114.7	144.4								29.6	27.5	149.2	198.1												2.0	2.0
Recovery of previously incurred costs																															2
Reduction in annual rates for its customers, percentage												2.00%		3.00%
Rate at which fuel costs will be monitored													2.00%			2.00%
Deferred compliance costs tolerance band					0.00%
Customer refund liability, current																$ 5