Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	1 Months Ended		1 Months Ended				1 Months Ended		6 Months Ended	1 Months Ended						3 Months Ended
	Jun. 30, 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] Cross State Air Pollution Rule [Member]	Jun. 30, 2012 WPL [Member] WPL S Edgewater Unit Five Purchase [Member]	May 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas Rate Request [Member]	Jun. 30, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related Request [Member]	Dec. 31, 2011 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member]	Jun. 30, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [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]	Jun. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] Tax Benefit Rider [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]	Jun. 30, 2011 IPL [Member] 2009 Test Year Retail Electric MN, Final Order [Member] Sutherland Four [Member]
Utility Rate Cases [Line Items]
Reduction in annual rates for its customers, amount				$ 13	$ 25					$ 36
Jointly owned utility plant, proportionate ownership share			25.00%
Increase in annual rates for its customers, amount						4			15			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%
Recovery of previously Iincurred costs																	2
Reduction in annual rates for its customers, percentage					2.00%
Deferred compliance costs tolerance band		0.00%
Rate at which fuel costs will be monitored	2.00%
Customer refund liability, current							$ 4