Utility Rate Cases (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended					1 Months Ended		3 Months Ended			1 Months Ended			12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Revenue Requirement Adjustment [Member]	Mar. 31, 2013 WPL [Member]	Mar. 31, 2012 WPL [Member]	Jul. 31, 2012 WPL [Member] 2013/2014 Test Period Retail Electric And Gas, Request [Member]	Dec. 31, 2012 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Oral Decision [Member]	Mar. 31, 2013 WPL [Member] 2013 Test Year Retail Electric Fuel Related, Request [Member]	Mar. 31, 2013 IPL [Member]	Mar. 31, 2012 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] Tax Benefit Rider [Member]	Nov. 30, 2012 IPL [Member] 2011 Test Year Retail Gas IA, Settlement [Member]	Dec. 31, 2013 Scenario, Forecast [Member] Revenue Requirement Adjustment [Member]
Utility Rate Cases [Line Items]
Reduction in annual rates for its customers, amount						$ 13	$ 29					$ 36
Proposed gas tax benefit rider, term												3 years
Increase in interim rates for its customers, amount											9
Increase in interim rates for its customers, percentage											3.00%
Increase in annual rates for its customers, amount													11
Increase in annual rates for its customers, percentage													4.00%
Revenue requirement adjustment	20.4	(26.5)							14.7	(33.6)				(24.0)
Electric revenue	$ 633.2	$ 572.4	$ 5.5	$ 283.0	$ 279.3				$ 350.2	$ 293.1
Reduction in annual rates for its customers, percentage							3.00%
Rate at which fuel costs will be monitored								2.00%