Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Deferred Tax Assets, Valuation allowance	$ 1.2		$ 18.5
Operating loss carryforwards, expiration dates	the state net operating losses carryforwards had expiration dates ranging from 2014 to 2031 with 99% expiring after 2020.
Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(752.2)		(407.6)
Deferred Tax Assets, Valuation allowance	1.2	[1]	18.5	[1]
Total deferred income tax assets	(751.0)		(389.1)
Deferred Tax Liabilities, Gross	2,320.4		1,801.9
Total deferred income tax liabilities	2,320.4		1,801.9
Total Deferred Tax (Assets) Liabilities, Net	1,568.2		1,394.3
Total deferred tax (assets) and liabilities, valuation allowance	1.2	[1]	18.5	[1]
Total deferred tax (assets) and liabilities	1,569.4		1,412.8
Other current assets	(22.8)		(21.5)
Deferred income taxes	1,592.2		1,434.3
IPL [Member]
Total deferred income tax assets	(396.5)		(213.4)
Total deferred income tax liabilities	1,319.9		1,049.2
Total deferred tax (assets) and liabilities	923.4		835.8
Other current assets	(13.5)		(13.2)
Deferred income taxes	936.9		849.0
WPL [Member]
Total deferred income tax assets	(208.1)		(82.2)
Total deferred income tax liabilities	874.6		(648.0)
Total deferred tax (assets) and liabilities	666.5		565.8
Other current assets	(6.0)		(4.6)
Deferred income taxes	672.5		570.4
Property [Member] | Alliant Energy Corporation [Member]
Deferred Tax Liabilities, Gross	1,926.4		1,419.8
Total Deferred Tax (Assets) Liabilities, Net	1,926.4		1,419.8
Property [Member] | IPL [Member]
Deferred Tax Liabilities, Gross	1,134.1		850.5
Total Deferred Tax (Assets) Liabilities, Net	1,134.1		850.5
Property [Member] | WPL [Member]
Deferred Tax Liabilities, Gross	697.1		494.8
Total Deferred Tax (Assets) Liabilities, Net	697.1		494.8
Deferred Portion Of Tax Gain On IPL's Electric Transmission Assets Sale [Member] | Alliant Energy Corporation [Member]
Deferred Tax Liabilities, Gross	75.5		100.3
Total Deferred Tax (Assets) Liabilities, Net	75.5		100.3
Deferred Portion Of Tax Gain On IPL's Electric Transmission Assets Sale [Member] | IPL [Member]
Deferred Tax Liabilities, Gross	75.5		100.3
Total Deferred Tax (Assets) Liabilities, Net	75.5		100.3
Investment In ATC [Member] | Alliant Energy Corporation [Member]
Deferred Tax Liabilities, Gross	93.8		83.9
Total Deferred Tax (Assets) Liabilities, Net	93.8		83.9
Investment In ATC [Member] | WPL [Member]
Deferred Tax Liabilities, Gross	93.8		83.9
Total Deferred Tax (Assets) Liabilities, Net	93.8		83.9
Pension And Other Postretirement Benefits Obligations [Member] | Alliant Energy Corporation [Member]
Deferred Tax Liabilities, Gross	91.9		51.1
Total Deferred Tax (Assets) Liabilities, Net	91.9		51.1
Pension And Other Postretirement Benefits Obligations [Member] | IPL [Member]
Deferred Tax Liabilities, Gross	67.6		43.9
Total Deferred Tax (Assets) Liabilities, Net	67.6		43.9
Pension And Other Postretirement Benefits Obligations [Member] | WPL [Member]
Deferred Tax Liabilities, Gross	49.6		32.1
Total Deferred Tax (Assets) Liabilities, Net	49.6		32.1
Investment Tax Credits [Member] | WPL [Member]
Deferred Tax Assets, Gross	(6.6)		(7.4)
Total Deferred Tax (Assets) Liabilities, Net	(6.6)		(7.4)
Regulatory Liability Flood Insurance [Member] | IPL [Member]
Deferred Tax Assets, Gross	(2.1)		(16.8)
Total Deferred Tax (Assets) Liabilities, Net	(2.1)		(16.8)
Regulatory Liability DAEC Sale [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(2.5)		(13.7)
Total Deferred Tax (Assets) Liabilities, Net	(2.5)		(13.7)
Regulatory Liability DAEC Sale [Member] | IPL [Member]
Deferred Tax Assets, Gross	(2.5)		(13.7)
Total Deferred Tax (Assets) Liabilities, Net	(2.5)		(13.7)
Customer Advances [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(13.7)		(15.0)
Total Deferred Tax (Assets) Liabilities, Net	(13.7)		(15.0)
Customer Advances [Member] | WPL [Member]
Deferred Tax Assets, Gross	(11.6)		(13.1)
Total Deferred Tax (Assets) Liabilities, Net	(11.6)		(13.1)
Net Operating Losses Carryforward State [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(39.9)		(19.2)
Total Deferred Tax (Assets) Liabilities, Net	(39.9)		(19.2)
Net Operating Losses Carryforward State [Member] | IPL [Member]
Deferred Tax Assets, Gross	(12.5)	[2]	(0.2)	[2]
Total Deferred Tax (Assets) Liabilities, Net	(12.5)	[2]	(0.2)	[2]
Net Operating Losses Carryforward State [Member] | WPL [Member]
Deferred Tax Assets, Gross	(5.1)	[2]
Total Deferred Tax (Assets) Liabilities, Net	(5.1)	[2]
Regulatory Liability-IPL's Electric Transmission Assets Sale [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(15.6)		(27.2)
Total Deferred Tax (Assets) Liabilities, Net	(15.6)		(27.2)
Regulatory Liability-IPL's Electric Transmission Assets Sale [Member] | IPL [Member]
Deferred Tax Assets, Gross	(15.6)		(27.2)
Total Deferred Tax (Assets) Liabilities, Net	(15.6)		(27.2)
Regulatory Liability Repairs Expenditures [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(71.8)		(60.9)
Total Deferred Tax (Assets) Liabilities, Net	(71.8)		(60.9)
Regulatory Liability Repairs Expenditures [Member] | IPL [Member]
Deferred Tax Assets, Gross	(71.8)		(60.9)
Total Deferred Tax (Assets) Liabilities, Net	(71.8)		(60.9)
Regulatory Liability - Mixed Service Costs Deduction [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(66.7)
Total Deferred Tax (Assets) Liabilities, Net	(66.7)
Regulatory Liability - Mixed Service Costs Deduction [Member] | IPL [Member]
Deferred Tax Assets, Gross	(66.7)
Total Deferred Tax (Assets) Liabilities, Net	(66.7)
Federal Credit Carryforward [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(107.4)		(79.8)
Total Deferred Tax (Assets) Liabilities, Net	(107.4)		(79.8)
Federal Credit Carryforward [Member] | IPL [Member]
Deferred Tax Assets, Gross	(25.0)	[2]	(12.4)	[2]
Total Deferred Tax (Assets) Liabilities, Net	(25.0)	[2]	(12.4)	[2]
Federal Credit Carryforward [Member] | WPL [Member]
Deferred Tax Assets, Gross	(26.1)	[2]	(11.5)	[2]
Total Deferred Tax (Assets) Liabilities, Net	(26.1)	[2]	(11.5)	[2]
Net Operating Losses Carryforward-Federal [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(336.1)		(93.7)
Total Deferred Tax (Assets) Liabilities, Net	(336.1)		(93.7)
Net Operating Losses Carryforward-Federal [Member] | IPL [Member]
Deferred Tax Assets, Gross	(154.4)	[3]	(41.3)	[3]
Total Deferred Tax (Assets) Liabilities, Net	(154.4)	[3]	(41.3)	[3]
Net Operating Losses Carryforward-Federal [Member] | WPL [Member]
Deferred Tax Assets, Gross	(141.1)	[3]	(38.2)	[3]
Total Deferred Tax (Assets) Liabilities, Net	(141.1)	[3]	(38.2)	[3]
Other Tax [Member] | Alliant Energy Corporation [Member]
Deferred Tax Assets, Gross	(98.5)		(98.1)
Deferred Tax Liabilities, Gross	132.8		146.8
Total Deferred Tax (Assets) Liabilities, Net	34.3		48.7
Other Tax [Member] | IPL [Member]
Deferred Tax Assets, Gross	(45.9)		(40.9)
Deferred Tax Liabilities, Gross	42.7		54.5
Total Deferred Tax (Assets) Liabilities, Net	(3.2)		13.6
Other Tax [Member] | WPL [Member]
Deferred Tax Assets, Gross	(17.6)		(12.0)
Deferred Tax Liabilities, Gross	34.1		37.2
Total Deferred Tax (Assets) Liabilities, Net	$ 16.5		$ 25.2
Federal [Member] | Alliant Energy Corporation [Member]
Operating loss carryforwards, expiration dates	2030
Federal [Member] | IPL [Member]
Operating loss carryforwards, expiration dates	2030
Federal [Member] | WPL [Member]
Operating loss carryforwards, expiration dates	2030
State [Member] | Alliant Energy Corporation [Member]
Operating loss carryforwards, expiration dates	2014	[4]
Federal State And Local [Member] | IPL [Member]
Federal credits, earliest expiration date	2022
Federal State And Local [Member] | WPL [Member]
Federal credits, earliest expiration date	2022
General Business Credits [Member] | Federal [Member] | Alliant Energy Corporation [Member]
Federal credits, earliest expiration date	2022
Alternative Minimum Tax [Member] | Federal [Member] | Alliant Energy Corporation [Member]
Federal credits, earliest expiration date	None

[1] Refer to "Wisconsin tax legislation" above for discussion of the reversal of valuation allowances related to state net operating loss carryforwards in 2011 as a result of a Wisconsin tax law change.

[2] Earliest expiration date is 2022.

[3] Earliest expiration date is 2030.

[4] At Dec. 31, 2011, the state net operating losses carryforwards had expiration dates ranging from 2014 to 2031 with 99% expiring after 2020. Due to the uncertainty of the realization of state net operating losses tax carryforwards, Alliant Energy had valuation allowances of $1.2 million and $18.5 million as of Dec. 31, 2011 and 2010, respectively. Refer to "Wisconsin tax legislation" above for discussion of the reversal of valuation allowances related to state net operating loss carryforwards in 2011 as a result of a Wisconsin tax law change.