Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended	3 Months Ended	9 Months Ended					12 Months Ended	3 Months Ended			9 Months Ended							9 Months Ended			1 Months Ended	9 Months Ended		3 Months Ended	12 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended	3 Months Ended		9 Months Ended			1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 2009 Test Year Retail Electric Request IA Final Order [Member] IPL Electric Transmission Assets Sale [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 Other Long-Term Liabilities And Deferred Credits [Member] Cross State Air Pollution Rule [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 Wind Project [Member]	Sep. 30, 2011 Alliant Energy Corporation [Member] SO2 Emission Allowances [Member]		Sep. 30, 2011 IPL [Member] SO2 Emission Allowances [Member]		Dec. 31, 2007 IPL [Member] SO2 Emission Allowances [Member]	Sep. 30, 2011 SO2 Emission Allowances [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 Cross State Air Pollution Rule [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 IPL [Member] Minnesota [Member] Whispering Willow-East Wind Project [Member]	Sep. 30, 2011 Alliant Energy Corporation [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2011 IPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2011 WPL [Member] Other Postretirement Benefit Plans [Member]	Sep. 30, 2011 IPL Electric Transmission Assets Sale [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 Energy Conservation Cost Recovery [Member] Alliant Energy And WPL [Member]	Sep. 30, 2011 WPL [Member] Bent Tree - Phase I Wind Project [Member]	Dec. 31, 2010 WPL [Member] Bent Tree - Phase I Wind Project [Member]	Sep. 30, 2011 Bent Tree - Phase I Wind Project [Member] Whispering Willow-East Wind Project [Member] IPL And WPL [Member] mW	Sep. 30, 2011 Bent Tree - Phase I Wind Project [Member]	Sep. 30, 2011 Bent Tree - Phase I Wind Project [Member] Alliant Energy And WPL [Member]	Jun. 30, 2011 IPL [Member] Sutherland Four Cancellation Costs [Member]	Sep. 30, 2011 Sutherland Four Cancellation Costs [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 IPL [Member] IPL's Duane Arnold Energy Center (DAEC) Sale[Member]	Mar. 31, 2011 WPL [Member] Green Lake And Fond Du Lac Counties Wind Site [Member]	Dec. 31, 2008 Vestas Wind Turbine Generators [Member] mW	Sep. 30, 2011 Alliant Energy Corporation [Member] Whispering Willow-East Wind Project [Member]	Sep. 30, 2011 IPL [Member] Whispering Willow-East Wind Project [Member]	Sep. 30, 2011 Whispering Willow-East Wind Project [Member] Alliant Energy And IPL [Member]	Sep. 30, 2011 Edgewater Unit Five Emission Controls [Member] Alliant Energy And WPL [Member]	Jun. 30, 2011 Alliant Energy Corporation [Member] Franklin County Wind Project [Member] mW	Sep. 30, 2011 IPL [Member] Franklin County Wind Project [Member]	Mar. 31, 2011 WPL [Member] WPL's Edgewater Unit 5 Purchase [Member]	Sep. 30, 2011 WPL [Member] WPL's Edgewater Unit 5 Ownership [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]	Dec. 31, 2010 IPL [Member]	Mar. 31, 2011 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]	Sep. 30, 2011 WPL [Member]	Sep. 30, 2010 WPL [Member]	Dec. 31, 2010 WPL [Member]
Recovery of plant cancellation costs																											$ 2
Unrecognized net actuarial losses that decreased regulatory assets			191.1	108.4													53.0	35.0	18.0									2.0															202.0	85.6					(10.9)	22.8
Credit to utility operation and maintenance																												2
Regulatory liabilities	904.0		904.0		800.1															3.0									23.0			26.0	26.0								713.8		713.8		627.9		190.2		190.2		172.2
Asset impairment loss																														5				8
Increase (decrease) in regulatory liabilities			158.5	18.6		(15.0)														5.0	14.0																						145.1	27.3					13.4	(8.7)
Long-term purchase commitment, amount							34	19	34	[1]	34	[1]	34	34	34																									1,484			790						471
Regulatory Assets	1,295.7		1,295.7		1,141.7										30.0																										1,002.8		1,002.8		799.6		292.9		292.9		342.1
Electric production fuel and energy purchases	215.3	238.8	590.0	642.4											2.0																										109.9	130.2	294.1	332.0			105.4	108.6	295.9	310.4
Utility - other operation and maintenance	147.1	143.4	476.6	446.1											2.0					5.0																					84.8	86.9	282.0	271.1			62.3	56.5	194.6	175.0
Recognized impairment charges, regulatory assets																																								7	2		2				5		5
Electric capacity of wind project																								400							500					100
Electric plant in service	8,103.9		8,103.9		7,676.8																	184.0	265.0															84.0			4,651.9		4,651.9		4,562.2		3,452.0		3,452.0		3,114.6
Capitalized cost			480.2	587.6																					435.0										60.0			38.0					232.3	272.5		38.0			247.9	315.1
Period which capitalized costs are depreciated over, in years																										30
Allowance for funds used during construction	2.8	5.0	8.6	14.4																		14.0													2.0						1.3	1.1	4.3	4.8			1.5	3.9	4.3	9.6
Equity method investment, ownership percentage																																						25.00%	100.00%
Comprehensive income	122.0	149.7	247.4	240.0																																					0	0	0	0			0	0	0	0
Proceeds from sale of wind project assets to affiliate																																					115.3						115.3
Disallowed recovery of project costs																																	8
Project costs allocated by jurisdiction																30
Amounts not probable of recovery															$ 4

[1] Refer to Note 1(b) for discussion of $34 million of charges recognized by Alliant Energy and IPL in the third quarter of 2011 for IPL's forward contracts to purchase SO2 emission allowances.