Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended			12 Months Ended				9 Months Ended
	Sep. 30, 2012 wind_projects	Sep. 30, 2011	Sep. 30, 2012 Alliant Energy [Member]	Sep. 30, 2011 Alliant Energy [Member]	Sep. 30, 2012 Alliant Energy [Member]	Sep. 30, 2011 Alliant Energy [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2012 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2012 WPL [Member]	Sep. 30, 2011 WPL [Member]	Mar. 31, 2012 State Apportionment [Member] Alliant Energy [Member]	Sep. 30, 2012 State Apportionment [Member] Alliant Energy [Member]	Mar. 31, 2012 State Apportionment [Member] IPL [Member]	Sep. 30, 2012 State Apportionment [Member] IPL [Member]	Mar. 31, 2012 State Apportionment [Member] WPL [Member]	Sep. 30, 2012 State Apportionment [Member] WPL [Member]	Sep. 30, 2012 Tax Benefit Rider [Member]	Sep. 30, 2011 Tax Benefit Rider [Member] Alliant Energy [Member]	Sep. 30, 2011 Tax Benefit Rider [Member] Alliant Energy [Member]	Sep. 30, 2012 Tax Benefit Rider [Member] IPL [Member]	Sep. 30, 2011 Tax Benefit Rider [Member] IPL [Member]	Sep. 30, 2012 Tax Benefit Rider [Member] IPL [Member]	Sep. 30, 2011 Tax Benefit Rider [Member] IPL [Member]	Sep. 30, 2011 Wisconsin Tax Legislation [Member]	Sep. 30, 2011 Wisconsin Tax Legislation [Member]	Dec. 31, 2011 Wisconsin Tax Legislation [Member]	Sep. 30, 2012 Cedar Ridge Wind Project [Member] WPL [Member] MW	Sep. 30, 2012 Bent Tree - Phase I Wind Project [Member] WPL [Member] MW	Sep. 30, 2012 Whispering Willow - East Wind Project [Member] IPL [Member] MW	Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] Alliant Energy [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] IPL [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] WPL [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] Resources [Member]	Sep. 30, 2012 Scenario, Forecast [Member] Tax Benefit Rider [Member] Alliant Energy [Member]	Sep. 30, 2012 Scenario, Forecast [Member] Tax Benefit Rider [Member] IPL [Member]	Sep. 30, 2012 Deferred income tax assets [Member]	Sep. 30, 2011 Deferred income tax assets [Member]	Sep. 30, 2012 Deferred income tax assets [Member] IPL [Member]	Sep. 30, 2011 Deferred income tax assets [Member] IPL [Member]	Sep. 30, 2012 Deferred income tax assets [Member] WPL [Member]	Sep. 30, 2011 Deferred income tax assets [Member] WPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate	35.00%
Deferred income tax expense															$ 15.2		$ 8.1		$ 7.0
Income tax rates for continuing operations			20.40%	23.50%	24.00%	16.60%	(12.60%)	(3.30%)	(5.60%)	(1.70%)	35.50%	34.40%	37.70%	33.00%		4.30%		6.90%		3.50%		9.20%	8.90%	26.50%			25.40%		5.80%									11.00%	34.50%
Regulatory liabilities	5.0	158.5							(27.5)	145.1			32.5	13.4							63.0			23.0	20.0	63.0	44.0
Wind projects generating production tax credits (in wind projects)	3
Electric capacity of wind projects generating production tax credits (in megawatts)																															68	200	200
Operating losses utilized																														368
Valuation allowance, deferred tax asset, change in amount																												19
Deferred income taxes																																								67.2	0.6	22.4	(3.1)	44.3	1.3
Increase in deferred income taxes noncurrent liabilities					244.5				102.5				136.8
Deductions in income tax return																																		$ 415	$ 114	$ 203	$ 98