Segments Of Business (Schedule Of Segment Of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Segment Reporting Information [Line Items]
Operating revenues	$ 879.2		$ 1,021.6	$ 819.5	$ 945.0	$ 832.6		$ 951.7	$ 741.6	$ 890.2	$ 3,665.3		$ 3,416.1		$ 3,427.3
Depreciation and amortization											323.8		291.3		273.6
Operating income (loss)	93.5		185.1	72.0	127.8	99.8		246.6	100.7	109.0	478.4		556.1		396.2
Interest expense, net of AFUDC											146.3		145.0		115.2
Loss on early extinguishment of debt															203.0
Equity (income)/loss from unconsolidated investments, net											(39.3)		(38.1)		(36.6)
Interest income and other											(4.3)		(4.0)		(4.9)
Income tax expense (benefit)											55.1		145.2		(9.3)
Income (loss) from continuing operations, net of tax											320.6		308.0		128.8
Income (loss) from discontinued operations, net of tax											1.3		(1.7)		0.9
Net income (loss)											321.9		306.3		129.7
Preferred dividends											18.3		18.7		18.7
Net income (loss) attributable to Alliant Energy common shareowners	57.0		122.0	51.1	73.5	47.3		149.1	47.8	43.4	303.6		287.6		111.0
Total assets	9,687.9					9,282.9					9,687.9		9,282.9		9,036.0
Investments in equity method subsidiaries	249.6					238.5					249.6		238.5		229.2
Construction and acquisition expenditures											673.4		866.9		1,202.6
IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	404.0		484.5	374.1	477.5	417.2		534.8	389.3	454.5	1,740.1		1,795.8		1,708.0
Depreciation and amortization											179.1		177.0		152.8
Operating income (loss)	26.3		116.5	18.5	47.1	22.7		155.7	45.2	38.3	208.4		261.9		221.7
Interest expense, net of AFUDC											72.9		76.7		42.5
Interest income and other											(0.2)		(0.5)		(0.8)
Income tax expense (benefit)											(3.6)		42.3		27.0
Net income (loss)	9.8		101.4	1.0	27.1	11.9		100.6	21.2	9.7	139.3		143.4		153.0
Preferred dividends											15.0		15.4		15.4
Net income (loss) attributable to Alliant Energy common shareowners	6.6		98.3	(2.3)	21.7	8.0		96.8	17.4	5.8	124.3		128.0		137.6
Total assets	5,093.5					4,937.6					5,093.5		4,937.6		4,892.2
Construction and acquisition expenditures											293.7		382.8		733.6
WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues	344.5		374.6	326.8	388.5	348.2		369.6	312.9	392.9	1,434.4		1,423.6		1,386.1
Depreciation and amortization											140.1		108.6		115.4
Operating income (loss)	73.2		86.9	45.5	74.8	70.1		86.7	52.3	69.7	280.4		278.8		167.0
Interest expense, net of AFUDC											73.7		66.1		69.1
Equity (income)/loss from unconsolidated investments, net											(38.7)		(37.8)		(37.0)
Interest income and other													(0.1)		(0.4)
Income tax expense (benefit)											81.9		98.3		45.8
Net income (loss)	42.7		51.4	25.0	44.4	35.1		50.0	30.2	37.0	163.5		152.3		89.5
Preferred dividends											3.3		3.3		3.3
Net income (loss) attributable to Alliant Energy common shareowners	41.9		50.6	24.1	43.6	34.3		49.2	29.3	36.2	160.2		149.0		86.2
Total assets	4,044.0					3,889.6					4,044.0		3,889.6		3,681.4
Investments in equity method subsidiaries	246.5					236.0					246.5		236.0		227.1
Construction and acquisition expenditures											314.4		450.5		508.4
Electric [Member]
Segment Reporting Information [Line Items]
Operating revenues											2,635.8		2,674.2		2,475.9
Depreciation and amortization											289.0		255.1		233.5
Operating income (loss)											444.2		489.8		342.2
Equity (income)/loss from unconsolidated investments, net											(38.7)		(37.8)		(37.0)
Total assets	7,524.5					7,227.2					7,524.5		7,227.2		6,867.6
Investments in equity method subsidiaries	246.5					236.0					246.5		236.0		227.1
Construction and acquisition expenditures											542.7		729.1		1,191.8
Electric [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues											1,408.3		1,464.3		1,315.6
Depreciation and amortization											161.3		157.1		130.3
Operating income (loss)											181.6		239.7		196.8
Total assets	4,208.2					4,025.3					4,208.2		4,025.3		3,976.6
Construction and acquisition expenditures											245.6		298.8		711.3
Electric [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues											1,227.5		1,209.9		1,160.3
Depreciation and amortization											127.7		98.0		103.2
Operating income (loss)											262.6		250.1		145.4
Equity (income)/loss from unconsolidated investments, net											(38.7)		(37.8)		(37.0)
Total assets	3,316.3					3,201.9					3,316.3		3,201.9		2,891.0
Investments in equity method subsidiaries	246.5					236.0					246.5		236.0		227.1
Construction and acquisition expenditures											297.1		430.3		480.5
Gas [Member]
Segment Reporting Information [Line Items]
Operating revenues											476.7		480.6		525.3
Depreciation and amortization											28.4		25.2		25.8
Operating income (loss)											47.8		53.4		52.3
Total assets	831.9					817.6					831.9		817.6		803.1
Construction and acquisition expenditures											38.0		39.9		45.2
Gas [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues											276.3		274.3		308.8
Depreciation and amortization											16.0		14.6		13.6
Operating income (loss)											20.6		23.6		27.7
Total assets	471.1					460.3					471.1		460.3		461.4
Construction and acquisition expenditures											21.2		20.0		17.5
Gas [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues											200.4		206.3		216.5
Depreciation and amortization											12.4		10.6		12.2
Operating income (loss)											27.2		29.8		24.6
Total assets	360.8					357.3					360.8		357.3		341.7
Construction and acquisition expenditures											16.8		19.9		27.7
Other Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues											62.0		64.6		92.9
Depreciation and amortization											1.8		5.3		8.9
Operating income (loss)											(3.2)		(2.5)		(5.8)
Total assets	781.1					782.4					781.1		782.4		902.9
Construction and acquisition expenditures											27.4		64.3		5.0
Other Utility [Member] | IPL [Member]
Segment Reporting Information [Line Items]
Operating revenues											55.5		57.2		83.6
Depreciation and amortization											1.8		5.3		8.9
Operating income (loss)											6.2		(1.4)		(2.8)
Total assets	414.2					452.0					414.2		452.0		454.2
Construction and acquisition expenditures											26.9		64.0		4.8
Utility Business [Member]
Segment Reporting Information [Line Items]
Operating revenues											3,174.5		3,219.4		3,094.1
Depreciation and amortization											319.2		285.6		268.2
Operating income (loss)											488.8		540.7		388.7
Interest expense, net of AFUDC											146.6		142.8		111.6
Equity (income)/loss from unconsolidated investments, net											(38.7)		(37.8)		(37.0)
Interest income and other											(0.2)		(0.6)		(1.2)
Income tax expense (benefit)											78.3		140.6		72.8
Income (loss) from continuing operations, net of tax											302.8		295.7		242.5
Net income (loss)											302.8		295.7		242.5
Preferred dividends											18.3		18.7		18.7
Net income (loss) attributable to Alliant Energy common shareowners											284.5		277.0		223.8
Total assets	9,137.5					8,827.2					9,137.5		8,827.2		8,573.6
Investments in equity method subsidiaries	246.5					236.0					246.5		236.0		227.1
Construction and acquisition expenditures											608.1		833.3		1,242.0
Other Segments [Member] | WPL [Member]
Segment Reporting Information [Line Items]
Operating revenues											6.5		7.4		9.3
Operating income (loss)											(9.4)		(1.1)		(3.0)
Total assets	366.9					330.4					366.9		330.4		448.7
Construction and acquisition expenditures											0.5		0.3		0.2
Non-Regulated [Member] | RMT [Member]
Segment Reporting Information [Line Items]
Operating revenues											443.9	[1]	154.0	[1]	294.1
Depreciation and amortization											2.8	[1]	4.0	[1]	2.4
Operating income (loss)											(34.9)	[1]	(3.9)	[1]	(4.9)
Interest expense, net of AFUDC											1.4	[1]	0.2	[1]	0.2
Interest income and other											(1.4)	[1]	0.6	[1]	(1.1)
Income tax expense (benefit)											(14.1)	[1]	(2.5)	[1]	(2.5)
Income (loss) from continuing operations, net of tax											(20.8)	[1]	(2.2)	[1]	(1.5)
Net income (loss)											(20.8)	[1]	(2.2)	[1]	(1.5)
Net income (loss) attributable to Alliant Energy common shareowners											(20.8)	[1]	(2.2)	[1]	(1.5)
Total assets	126.3	[1]				90.7	[1]				126.3	[1]	90.7	[1]	63.6
Construction and acquisition expenditures											0.7	[1]	1.4	[1]	8.2
Non-Regulated [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues											46.9		42.7		39.1
Depreciation and amortization											1.8		1.7		3.0
Operating income (loss)											24.5		19.3		12.4
Interest expense, net of AFUDC											(1.7)		2.0		3.4
Loss on early extinguishment of debt															203.0
Equity (income)/loss from unconsolidated investments, net											(0.6)		(0.3)		0.4
Interest income and other											(2.7)		(4.0)		(2.6)
Income tax expense (benefit)											(9.1)		7.1		(79.6)
Income (loss) from continuing operations, net of tax											38.6		14.5		(112.2)
Income (loss) from discontinued operations, net of tax											1.3		(1.7)		0.9
Net income (loss)											39.9		12.8		(111.3)
Net income (loss) attributable to Alliant Energy common shareowners											39.9		12.8		(111.3)
Total assets	424.1					365.0					424.1		365.0		398.8
Investments in equity method subsidiaries	3.1					2.5					3.1		2.5		2.1
Construction and acquisition expenditures											$ 64.6		$ 32.2		$ (47.6)

[1] RMT's operating revenues and expenses increased significantly in 2011 primarily due to increased demand for construction management services for large wind and solar projects. RMT's operating loss and net loss in 2011 was largely driven by losses associated with certain large solar projects as a result of issues with certain of its subcontractors engaged to complete the solar projects. Schedule delays, abandonment of work by the original subcontractor and the need to hire additional subcontractors to complete the work in a timely manner resulted in significant additional costs for RMT in 2011.