Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified	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			12 Months Ended		1 Months Ended		12 Months Ended	1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 2012 Test Year Retail Electric Fuel Related Request Final Order [Member]	Dec. 31, 2012 WPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Dec. 31, 2012 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member]	May 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas Rate Request [Member]	Dec. 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas Rate Request [Member] WPL's Edgewater Unit 5 Purchase [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]	Dec. 31, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member]	Jun. 30, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member]	Dec. 31, 2010 WPL [Member] 2010 Test Year Retail Electric Fuel Related Request [Member]	Dec. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related Request [Member]	Dec. 31, 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]	Dec. 31, 2012 WPL [Member] 2012 Test Year Retail Electric Fuel Related Request Final Order [Member]	Dec. 31, 2012 IPL [Member]	Dec. 31, 2011 IPL [Member]	Dec. 31, 2010 IPL [Member]	Nov. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [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]	Dec. 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member]	Dec. 31, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Request [Member] Tax Benefit Rider [Member]	Feb. 28, 2011 IPL [Member] 2009 Test Year Retail Electric IA, Request [Member]	Mar. 31, 2010 IPL [Member] 2009 Test Year Retail Electric IA, Request [Member]	Dec. 31, 2011 IPL [Member] 2009 Test Year Retail Electric IA, Request [Member]	Jul. 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]
Utility Rate Cases [Line Items]
Reduction in annual rates for its customers, amount									$ 13																		$ 36
Jointly owned utility plant, proportionate ownership share								25.00%		25.00%
Increase in annual rates for its customers, amount												8						4					11					114				11	8	2	1
Increase in annual rates for its customers, percentage												1.00%											4.00%					10.00%
Increase in interim rates for its customers, amount													9	9	5									9					119		14
Decrease in annual electric retail rates											1					29
Reduction in interim rates for its customers, amount															5
Requested rate increase, adjustment period subsequent to test year																									12 months				12 months
Proposed gas tax benefit rider, term																											3 years
Increase in interim rates for its customers, percentage																								3.00%
Gas revenue	396.3	476.7	480.6		169.6	200.4	206.3													226.7	276.3	274.3				5.0
Revenue refunded to customers, including interest																														5
Decrease in annual rate for its customers, percentage																3.00%
Rate at which fuel costs will be monitored				2.00%													2.00%
Refunds anticipated to be used to offset fuel related cost changes in 2014																			$ 11