Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended										3 Months Ended
	Sep. 30, 2013 wind_projects	Sep. 30, 2012	Sep. 30, 2013 Deferred income tax assets [Member]	Sep. 30, 2013 IPL [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2013 IPL [Member] Whispering Willow - East Wind Project [Member] MW	Sep. 30, 2013 WPL [Member]	Sep. 30, 2013 WPL [Member] Cedar Ridge Wind Project [Member] MW	Sep. 30, 2013 WPL [Member] Bent Tree - Phase I Wind Project [Member] MW	Sep. 30, 2013 WPL [Member] Deferred income tax assets [Member]	Mar. 31, 2012 State Apportionment [Member]	Mar. 31, 2012 State Apportionment [Member] IPL [Member]	Mar. 31, 2012 State Apportionment [Member] WPL [Member]
Income Tax [Line Items]
Wind projects generating production tax credits (in wind projects)	3
Electric capacity of wind projects generating production tax credits (in megawatts)						200		68	200
Deferred income tax expense											$ 15	$ 8	$ 7
Increase (decrease) current deferred tax assets	(75.4)	(90.5)	(39.0)	(71.0)	(88.9)					(40.7)
Increase (decrease) non-current deferred tax liabilities	$ 129.7			$ 73.7			$ 41.3