Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended																		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012 Alliant Energy Corporation [Member]	Mar. 31, 2011 Alliant Energy Corporation [Member]	Mar. 31, 2012 IPL [Member]	Mar. 31, 2011 IPL [Member]	Mar. 31, 2012 WPL [Member]	Mar. 31, 2011 WPL [Member]	Mar. 31, 2012 State Apportionment [Member] Alliant Energy Corporation [Member]	Mar. 31, 2012 State Apportionment [Member] IPL [Member]	Mar. 31, 2012 State Apportionment [Member] WPL [Member]	Mar. 31, 2011 Tax Benefit Rider [Member]	Mar. 31, 2011 Tax Benefit Rider [Member] Alliant Energy Corporation [Member]	Mar. 31, 2012 Tax Benefit Rider [Member] IPL [Member]	Mar. 31, 2011 Tax Benefit Rider [Member] IPL [Member]	Mar. 31, 2012 Cedar Ridge Wind Project [Member] MW	Mar. 31, 2012 Whispering Willow-East Wind Project [Member] MW	Mar. 31, 2012 Bent Tree - Phase I Wind Project [Member] MW	Dec. 31, 2012 Scenario, Forecast [Member] Bonus Depreciation 2012 Return [Member] Alliant Energy Corporation [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] Tax Benefit Rider [Member] Alliant Energy Corporation [Member]	Dec. 31, 2012 Scenario, Forecast [Member] Tax Benefit Rider [Member] IPL [Member]
Income Tax [Line Items]
Statutory federal income tax rate	35.00%
Deferred income tax expense									$ 15.2	$ 8.1	$ 7.0
Increase (Decrease) in deferred income taxes noncurrent liabilities			46		18		26
Income tax rates for continuing operations			39.00%	22.30%	125.40%	5.90%	44.10%	32.50%	21.40%	137.30%	12.30%		8.90%		22.90%							12.20%	37.50%
Regulatory liabilities	(26.5)	159.2			(33.6)	159.0						20.0		20.0	7.0
Wattage capacity of projects generating production tax credits																68	200	200
Deductions in income tax return																			$ 418	$ 114	$ 203