Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2007 Alliant Energy And IPL [Member]	Dec. 31, 2011 Alliant Energy Corporation [Member]	Dec. 31, 2010 Alliant Energy Corporation [Member]	Dec. 31, 2009 Alliant Energy Corporation [Member]	Dec. 31, 2011 IPL [Member]	Dec. 31, 2010 IPL [Member]	Dec. 31, 2009 IPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Dec. 31, 2009 WPL [Member]	Dec. 31, 2010 Federal Healthcare Legislation [Member] Alliant Energy Corporation [Member]	Dec. 31, 2010 Federal Healthcare Legislation [Member] IPL [Member]	Dec. 31, 2010 Federal Healthcare Legislation [Member] WPL [Member]	Dec. 31, 2011 Bonus Depreciation 2011 Return [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 Bonus Depreciation 2011 Return [Member] IPL [Member]	Dec. 31, 2011 Bonus Depreciation 2011 Return [Member] WPL [Member]	Dec. 31, 2010 Mixed Service Costs 2010 Return [Member] Alliant Energy Corporation [Member]	Dec. 31, 2010 Mixed Service Costs 2010 Return [Member] IPL [Member]	Dec. 31, 2010 Mixed Service Costs 2010 Return [Member] WPL [Member]	Dec. 31, 2011 Mixed Service Costs [Member] Alliant Energy And IPL [Member]	Dec. 31, 2011 Mixed Service Costs [Member] Current Liabilities [Member] Alliant Energy And IPL [Member]	Dec. 31, 2011 Mixed Service Costs [Member] Non-Current Liabilities [Member] Alliant Energy And IPL [Member]	Dec. 31, 2011 Wisconsin Tax Legislation [Member]	Dec. 31, 2009 Wisconsin Senate Bill Sixty Two Net Income Tax Benefits [Member] Alliant Energy Corporation [Member]	Dec. 31, 2009 Wisconsin Senate Bill Sixty Two Net Income Tax Benefits [Member] IPL [Member]	Dec. 31, 2009 Wisconsin Senate Bill Sixty Two Net Income Tax Benefits [Member] WPL [Member]	Dec. 31, 2011 State Income Taxes [Member] Alliant Energy And IPL [Member]	Dec. 31, 2010 State Income Taxes [Member] Alliant Energy And IPL [Member]	Dec. 31, 2009 State Income Taxes [Member] Alliant Energy And IPL [Member]	Dec. 31, 2010 Audit Completion Tax Rate Decrease [Member] Alliant Energy Corporation [Member]	Dec. 31, 2010 Audit Completion Tax Rate Decrease [Member] IPL [Member]	Dec. 31, 2010 Audit Completion Income Tax Benefits [Member] Alliant Energy Corporation [Member]	Dec. 31, 2010 Audit Completion Income Tax Benefits [Member] IPL [Member]	Dec. 31, 2011 Whispering Willow-East Wind Project [Member] mW	Dec. 31, 2011 Cedar Ridge Wind Project [Member] mW	Dec. 31, 2011 Bent Tree - Phase I Wind Project [Member] mW	Dec. 31, 2011 Repairs Expenditures [Member] Internal Revenue Service (IRS) [Member] Alliant Energy Corporation [Member]	Dec. 31, 2011 Repairs Expenditures [Member] Internal Revenue Service (IRS) [Member] IPL [Member]	Dec. 31, 2011 Repairs Expenditures [Member] Internal Revenue Service (IRS) [Member] WPL [Member]
Decrease in state taxes																														$ 6	$ 12	$ 2
Operating losses utilized																										368
State net operating loss carryforwards, valuation allowances																										19.0
Group shared net operating loss carryforwards, years utilized	20
Deferred income tax expense														7	4	3
Income tax rates for continuing operations					14.70%	32.00%	(7.80%)	(2.70%)	22.80%	15.00%	33.40%	39.20%	33.90%																				1.40%	2.70%
Unitary return in percentage	50.00%
Income taxes benefit from changes in state income tax rates																											40	33	2						7	5
Projects generating production tax credits																																					200	68	200
Defined Benefit Plan Contributions By Employer					126.0			61.0			52.0
Deductions in income tax return																	572	194	334	247	141	106
Taxable gain sale of electric transmission assets				527
Denied amount of tax return deduction																																								503	212	291
Regulatory liabilities	168.3	8.4	136.7		61.0			156.3	17.5	158.9	12.0	(9.1)	(22.2)											32.0	134.0
Increases in regulatory assets	$ 413.1	$ 20.8	$ 163.9					$ 328.8	$ 49.5	$ 109.7	$ 84.3	$ (28.7)	$ 54.2										$ 166.0