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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	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
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 Alliant Energy And IPL [Member] Current Liabilities [Member] Mixed Service Costs [Member]	Sep. 30, 2011 Alliant Energy And IPL [Member] Non-Current Liabilities [Member] Mixed Service Costs [Member]	Sep. 30, 2011 IPL [Member] Whispering Willow-East Wind Project [Member] mW	Sep. 30, 2011 WPL [Member] Cedar Ridge Wind Project [Member] mW	Sep. 30, 2011 WPL [Member] Bent Tree - Phase I Wind Project [Member] mW	Sep. 30, 2010 IPL [Member] Federal Healthcare Legislation Tax Rate Increase [Member]	Sep. 30, 2010 WPL [Member] Federal Healthcare Legislation Tax Rate Increase [Member]	Sep. 30, 2010 Federal Healthcare Legislation Tax Rate Increase [Member]	Sep. 30, 2011 IPL [Member] Tax Benefit Rider Tax Rate Decrease [Member]	Sep. 30, 2011 Tax Benefit Rider Tax Rate Decrease [Member]	Sep. 30, 2011 IPL [Member] Bonus Depreciation 2011 Return [Member]	Sep. 30, 2011 WPL [Member] Bonus Depreciation 2011 Return [Member]	Sep. 30, 2011 Bonus Depreciation 2011 Return [Member]	Sep. 30, 2011 IPL [Member] Mixed Service Costs 2010 Return [Member]	Sep. 30, 2011 WPL [Member] Mixed Service Costs 2010 Return [Member]	Sep. 30, 2011 Mixed Service Costs 2010 Return [Member]	Sep. 30, 2011 Alliant Energy And IPL [Member] Mixed Service Costs [Member]	Mar. 31, 2010 IPL [Member] Federal Health Care Legislation [Member]	Mar. 31, 2010 WPL [Member] Federal Health Care Legislation [Member]	Mar. 31, 2010 Federal Health Care Legislation [Member]	Sep. 30, 2010 Alliant Energy Corporation [Member] Federal Tax Audits [Member]	Sep. 30, 2010 Alliant Energy Corporation [Member] Federal Tax Audits [Member]	Sep. 30, 2010 IPL [Member] Federal Tax Audits [Member]	Sep. 30, 2010 IPL [Member] Federal Tax Audits [Member]	Jun. 30, 2011 Wisconsin Tax Legislation [Member]	Sep. 30, 2011 Wisconsin Tax Legislation [Member]	Sep. 30, 2011 Alliant Energy Corporation [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2010 IPL [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2010 IPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]
Federal statutory rate			35.00%
Deferred income tax expense																						$ 3.7	$ 3.1	$ 7.1
Income tax rates for continuing operations	21.30%	29.20%	15.10%	32.20%						2.00%	1.60%	1.90%	25.40%	8.70%											3.00%	1.70%	3.80%	2.80%		6.20%		(3.30%)	25.80%	(1.70%)	27.80%	34.40%	37.30%	33.00%	37.70%
Net income tax benefits	34.1	64.1	46.1	121.8																					6.5		5.1					(3.2)	35.0	(2.2)	50.7	26.9	29.8	59.6	71.0
Projects generating production tax credits							200	68	200
Increase decrease in deferred income taxes noncurrent liabilities																															127			61				84
Deductions in income tax return															220	360	580	141	106	247
Increase (decrease) in regulatory liabilities			158.5	18.6	21.0	148.0																												145.1	27.3			13.4	(8.7)
Increase (decrease) in regulatory assets			191.1	108.4																	169.0													202.0	85.6			(10.9)	22.8
Operating losses utilized																														368
Deferred tax asset valuation allowances related to state net operating loss																													$ 18.9