Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended			3 Months Ended	6 Months Ended				3 Months Ended	12 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended		6 Months Ended				1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 WPL [Member]	Jun. 30, 2011 WPL [Member]	Jun. 30, 2012 WPL [Member]	Jun. 30, 2011 WPL [Member]	Jun. 30, 2012 IPL [Member]	Jun. 30, 2011 IPL [Member]	Jun. 30, 2012 IPL [Member]	Jun. 30, 2011 IPL [Member]	Jun. 30, 2011 Sutherland Four [Member] Alliant Energy and IPL [Member]	Jun. 30, 2011 Sutherland Four [Member] Alliant Energy and IPL [Member]	Jun. 30, 2011 IPL Electric Transmission Assets Sale [Member] Alliant Energy and IPL [Member]	Jun. 30, 2012 WPL S Edgewater Unit Five Purchase [Member] WPL [Member]	Jun. 30, 2012 WPL 2013/2014 Test Period Retail Electric And Gas Final Order [Member] Alliant Energy and WPL [Member]	Jun. 30, 2012 Edgewater Unit 5 Emission Controls [Member] Alliant Energy and WPL [Member]	Jun. 30, 2012 Columbia Units 1 and 2 Emission Controls Project [Member] Alliant Energy and WPL [Member]	Jun. 30, 2012 Vestas Wind Turbine Generators [Member] MW	Jun. 30, 2012 Franklin County Wind Project [Member] MW	Jun. 30, 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	Jun. 30, 2012 Whispering Willow - East Wind Project [Member] IPL [Member]	Jun. 30, 2011 Whispering Willow - East Wind Project [Member] Alliant Energy and IPL [Member]	Jun. 30, 2012 Tax Benefit Rider [Member]	Jun. 30, 2012 Tax Benefit Rider [Member] IPL [Member]	Jun. 30, 2011 Tax Benefit Rider [Member] IPL [Member]	Jun. 30, 2012 Tax Benefit Rider [Member] IPL [Member]	Jun. 30, 2011 Tax Benefit Rider [Member] IPL [Member]	Jun. 30, 2012 Franklin County Wind Project [Member]	Jun. 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]	Jun. 30, 2012 Scenario, Forecast [Member] Electric Generation Equipment [Member] WPL [Member]	Jun. 30, 2012 Scenario, Forecast [Member] Electric Distribution [Member] WPL [Member]	Jun. 30, 2012 Scenario, Forecast [Member] Gas Transmission [Member] WPL [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Recovery of direct costs														$ 2
Increase in regulatory assets			19.6	110.3				7.5	(23.0)			12.1	133.3		2.0			5.0
Credit to utility other operation and maintenance															2			5
Regulatory liabilities			(43.5)	165.9								(61.5)	158.0			5.0											40.0	20.0	17.0	40.0	24.0
Utilities operating expense, maintenance and operations	137.9	168.9	287.9	329.5		54.1	68.4	117.2	132.3	83.8	100.5	170.7	197.2			5.0
Amount of impairment to carrying amount of regulatory assets		6		6			5		5		1		1
Jointly owned utility plant, proportionate ownership share																	25.00%
Capitalized expenditures			247.3	338.1				137.4	174.6			109.9	163.5						98.0	53.0
Allowance for funds used during construction	4.8	2.7	8.6	5.8		3.1	1.1	5.4	2.8	1.7	1.6	3.2	3.0						6.0	1.0
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																																				3.60%	2.70%	2.50%
Alliant Energy Corporate Services, Inc. and other	302.4		302.4		270.6																											183.0
Interest costs, capitalized during period																																6
Alliant Energy Corporate Services, Inc. and non-regulated businesses			$ 75.1	$ 20.6																														$ 48.0