Summary Of Significant Accounting Policies (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				9 Months Ended		1 Months Ended	9 Months Ended			9 Months Ended					3 Months Ended	12 Months Ended	9 Months Ended			3 Months Ended		9 Months Ended				1 Months Ended	3 Months Ended	9 Months Ended					12 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2012 WPL [Member]	Sep. 30, 2011 WPL [Member]	Dec. 31, 2011 WPL [Member]	Dec. 31, 2010 WPL [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2011 IPL [Member]	Sep. 30, 2012 IPL [Member]	Sep. 30, 2011 IPL [Member]	Dec. 31, 2011 IPL [Member]	Dec. 31, 2010 IPL [Member]	Sep. 30, 2012 George Neal Units 3 And 4 [Member] Alliant Energy and IPL [Member]	Sep. 30, 2012 Ottumwa Unit 1 [Member] Alliant Energy and IPL [Member]	Aug. 31, 2011 Sutherland 4 [Member] Alliant Energy and IPL [Member]	Sep. 30, 2011 Sutherland 4 [Member] Alliant Energy and IPL [Member]	Sep. 30, 2011 IPL Electric Transmission Assets Sale [Member] Alliant Energy and IPL [Member]	Sep. 30, 2012 WPL's Edgewater Unit 5 Purchase [Member] WPL [Member]	Sep. 30, 2012 WPL 2013/2014 Test Period Retail Electric And Gas Final Order [Member] Alliant Energy and WPL [Member]	Sep. 30, 2012 Edgewater Unit 5 Emission Controls [Member] Alliant Energy and WPL [Member]	Sep. 30, 2012 Columbia Units 1 And 2 [Member] Alliant Energy and WPL [Member]	Sep. 30, 2012 Vestas Wind Turbine Generators [Member] MW	Sep. 30, 2012 Franklin County Wind Project [Member] MW	Mar. 31, 2011 Green Lake and Fond du Lac Counties Wind Site [Member] WPL [Member]	Dec. 31, 2009 Green Lake and Fond du Lac Counties Wind Site [Member] WPL [Member] MW	Sep. 30, 2012 Whispering Willow - East Wind Project [Member] IPL [Member]	Sep. 30, 2011 Whispering Willow - East Wind Project [Member] Alliant Energy and IPL [Member]	Sep. 30, 2012 Tax Benefit Rider [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, 2012 Franklin County Wind Project [Member]	Sep. 30, 2012 Franklin County Wind Project [Member] Bent Tree - Phase I Wind Project [Member] IPL and WPL [Member] MW	Apr. 30, 2012 Corporate Headquarters [Member]	Jun. 30, 2011 Franklin County Wind Project [Member] IPL [Member]	Sep. 30, 2012 Scenario, Forecast [Member] WPL [Member]	Sep. 30, 2012 Scenario, Forecast [Member] IPL [Member]	Sep. 30, 2012 Scenario, Forecast [Member] Electric Generation Equipment [Member] WPL [Member]	Sep. 30, 2012 Scenario, Forecast [Member] Electric Distribution [Member] WPL [Member]	Sep. 30, 2012 Scenario, Forecast [Member] Gas Transmission [Member] WPL [Member]	Dec. 31, 2007 SO2 emission allowances [Member] IPL [Member]	Sep. 30, 2011 Cross State Air Pollution Rule [Member] Alliant Energy and IPL [Member]	Sep. 30, 2012 IPL's tax benefit rider [Member] Alliant Energy and IPL [Member]	Sep. 30, 2012 Energy conservation cost recovery [Member] Alliant Energy and WPL [Member]	Sep. 30, 2012 George Neal Unit 3 [Member] IPL [Member]	Sep. 30, 2012 Columbia Units 1 And 2 [Member] WPL [Member]	Sep. 30, 2012 George Neal Unit 4 [Member] IPL [Member]	Sep. 30, 2012 Ottumwa Unit 1 [Member] IPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Minimum future commitments																																											$ 355	$ 605				$ 34	$ 34
Regulatory assets	1,518.4		1,518.4		1,495.0		367.4		367.4		377.7		1,151.0		1,151.0		1,117.3																																30.0
Amounts not probable of recovery																																																	4
Electric production fuel and energy purchases	221.6	215.3	550.4	590.0			104.9	105.4	277.6	295.9			116.7	109.9	272.8	294.1																																	2.0
Utilities operating expense, maintenance and operations	144.7	147.1	432.6	476.6			58.3	62.3	175.5	194.6			86.4	84.8	257.1	282.0							5.0																										2.0
Recovery of direct costs																					2
Increase (decrease) in regulatory assets			73.5	191.1					15.8	(10.9)					57.7	202.0						2.0			5.0
Credit to utility other operation and maintenance																						2			5
Regulatory liabilities			5.0	158.5					32.5	13.4					(27.5)	145.1							5.0											63.0	23.0	20.0	63.0	44.0												22.0	15.0
Amount of impairment to carrying amount of regulatory assets		7		7				5		5				2		2
Jointly owned utility plant, proportionate ownership share																								25.00%																												28.00%	46.20%	25.695%	48.00%
Capitalized expenditures			412.7	480.2					218.1	247.9					194.6	232.3			53.0	52.0						116.0	90.0
Allowance for funds used during construction	5.8	2.8	14.4	8.6			3.7	1.5	9.1	4.3			2.1	1.3	5.3	4.3			1.0	1.0						8.0	2.0
Electric capacity of wind project (in megawatts)																												500	100											400
Proceeds from sale of wind project assets to affiliate															0	115.3																										115.3
Public utilities, property, plant and equipment, amount of indirect disallowance of costs of recently completed plants																																8
Project costs allocated by jurisdiction																																30
Asset impairment loss																														5			8
Purchase of development rights (in megawatts)																															100
Public utilities, property, plant and equipment, disclosure of composite depreciation rate for plants in service																																													3.40%	2.70%	2.50%
Non-regulated Generation capitalized expenditures, less accumulated depreciation	326.5		326.5		270.6																																		198.0
Interest costs, capitalized during period																																							8
Asset retirement obligations	97.2	90.6	97.2	90.6	91.1	75.9	42.9	34.5	42.9	34.5	34.9	32.3	45.9	56.1	45.9	56.1	56.2	43.6																					8.0
Alliant Energy Corporate Services, Inc. and non-regulated businesses capitalized expenditures			$ 106.3	$ 46.1																																					$ 48.0