Benefit Plans (Fair Value Of Plan Assets By Asset Category And Fair Value Hierarchy Level) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	$ 897.4		$ 823.0		$ 776.4
Total pension plan assets	897.4		823.0
Defined Benefit Pension Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	897.4		823.0
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	298.7		256.9		242.8
Total pension plan assets	298.7		256.9
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	289.6		255.2		236.6
Total pension plan assets	289.6		255.2
Defined Benefit Pension Plans [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | Cash And Equivalents [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	117.5		11.1
Defined Benefit Pension Plans [Member] | Cash And Equivalents [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	39.1		3.5
Defined Benefit Pension Plans [Member] | Cash And Equivalents [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	37.9		3.4
Defined Benefit Pension Plans [Member] | Cash And Equivalents [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	117.5		11.1
Defined Benefit Pension Plans [Member] | Cash And Equivalents [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	39.1		3.5
Defined Benefit Pension Plans [Member] | Cash And Equivalents [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	37.9		3.4
Defined Benefit Pension Plans [Member] | Cash And Equivalents [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	9.3		17.3
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	3.1		5.4
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	3.0		5.4
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.7		3.0
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.6		0.9
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.5		1.0
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.8		11.5
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.9		3.6
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.9		3.5
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.8		2.8
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 3 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.9
Defined Benefit Pension Plans [Member] | Securities Lending Invested Collateral [Member] | Level 3 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.9
Defined Benefit Pension Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	110.7		118.4
Defined Benefit Pension Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	36.8		37.0
Defined Benefit Pension Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	35.7		36.7
Defined Benefit Pension Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	110.7		118.4
Defined Benefit Pension Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	36.8		37.0
Defined Benefit Pension Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	35.7		36.7
Defined Benefit Pension Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	91.6		100.4
Defined Benefit Pension Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	30.5		31.4
Defined Benefit Pension Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.6		31.2
Defined Benefit Pension Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	91.6		100.4
Defined Benefit Pension Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	30.5		31.4
Defined Benefit Pension Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.6		31.2
Defined Benefit Pension Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	91.5		101.0
Defined Benefit Pension Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	30.5		31.5
Defined Benefit Pension Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.5		31.3
Defined Benefit Pension Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	91.5		101.0
Defined Benefit Pension Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	30.5		31.5
Defined Benefit Pension Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.5		31.3
Defined Benefit Pension Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	25.7		29.0
Defined Benefit Pension Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	8.6		9.0
Defined Benefit Pension Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	8.3		9.0
Defined Benefit Pension Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	25.7		29.0
Defined Benefit Pension Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	8.6		9.0
Defined Benefit Pension Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	8.3		9.0
Defined Benefit Pension Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	21.7		16.6
Defined Benefit Pension Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	7.2		5.2
Defined Benefit Pension Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	7.0		5.2
Defined Benefit Pension Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	21.7		16.6
Defined Benefit Pension Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	7.2		5.2
Defined Benefit Pension Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	7.0		5.2
Defined Benefit Pension Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	126.4		142.3
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	42.1		44.4
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	40.8		44.1
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	65.4		76.0
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	21.8		23.7
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	21.1		23.6
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	61.0		66.3
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	20.3		20.7
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	19.7		20.5
Defined Benefit Pension Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	30.4		34.1
Defined Benefit Pension Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	10.1		10.7
Defined Benefit Pension Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	9.8		10.6
Defined Benefit Pension Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	30.4		34.1
Defined Benefit Pension Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	10.1		10.7
Defined Benefit Pension Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	9.8		10.6
Defined Benefit Pension Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	57.1		54.2
Defined Benefit Pension Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	19.0		16.9
Defined Benefit Pension Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	18.4		16.8
Defined Benefit Pension Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	57.1		54.2
Defined Benefit Pension Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	19.0		16.9
Defined Benefit Pension Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	18.4		16.8
Defined Benefit Pension Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	87.8		78.9
Defined Benefit Pension Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.2		24.6
Defined Benefit Pension Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	28.3		24.5
Defined Benefit Pension Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	87.8		78.9
Defined Benefit Pension Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.2		24.6
Defined Benefit Pension Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	28.3		24.5
Defined Benefit Pension Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	146.7		142.7
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	48.8		44.5
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	47.4		44.2
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.2
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	146.5		142.5
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	48.7		44.5
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	47.3		44.2
Defined Benefit Pension Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | Accrued Investment Income [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		1.2
Defined Benefit Pension Plans [Member] | Accrued Investment Income [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.4		0.4
Defined Benefit Pension Plans [Member] | Accrued Investment Income [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.3		0.3
Defined Benefit Pension Plans [Member] | Accrued Investment Income [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	916.4		846.0
Defined Benefit Pension Plans [Member] | Subtotal [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	305.0		264.1
Defined Benefit Pension Plans [Member] | Subtotal [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	295.7		262.4
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	350.6		259.4
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	116.7		81.0
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	113.1		80.5
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	564.0		583.8
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	187.7		182.2
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	182.0		181.0
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.8		2.8
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 3 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.9
Defined Benefit Pension Plans [Member] | Subtotal [Member] | Level 3 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.9
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	120.4		122.7		102.4
Other Postretirement Benefits Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	120.4		122.7
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	74.7		77.5		68.4
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	25.1		25.0		18.8
Other Postretirement Benefits Plans [Member] | Cash And Equivalents [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	14.0		15.4
Other Postretirement Benefits Plans [Member] | Cash And Equivalents [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.1		3.5
Other Postretirement Benefits Plans [Member] | Cash And Equivalents [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	10.5		10.7
Other Postretirement Benefits Plans [Member] | Cash And Equivalents [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	14.0		15.4
Other Postretirement Benefits Plans [Member] | Cash And Equivalents [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.1		3.5
Other Postretirement Benefits Plans [Member] | Cash And Equivalents [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	10.5		10.7
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.4		0.8
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.5
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.2
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.2
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.1
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.1
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.5
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.3
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross			0.1
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.1
Other Postretirement Benefits Plans [Member] | Securities Lending Invested Collateral [Member] | Level 3 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross			0.1
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	37.1		38.6
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.3		30.7
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.2		1.3
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	37.1		38.6
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	29.3		30.7
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.2		1.3
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.4		2.7
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.8		1.0
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		1.1
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.4		2.7
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.8		1.0
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		1.1
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.4		2.7
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.8		1.0
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		1.1
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.4		2.7
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.8		1.0
Other Postretirement Benefits Plans [Member] | U.S. Large Cap Growth [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		1.1
Other Postretirement Benefits Plans [Member] | U.S. Mid Cap Core [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	17.8		18.6
Other Postretirement Benefits Plans [Member] | U.S. Mid Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	17.8		18.6
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Core [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.7		3.8
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Core [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	14.7		15.3
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Core [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.7		3.8
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.7		3.8
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	14.7		15.3
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Core [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.7		3.8
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.8
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.3
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.3		0.3
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.8
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.3
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Value [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.3		0.3
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.5		0.4
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.2
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.2
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.5		0.4
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.2
Other Postretirement Benefits Plans [Member] | U.S. Small Cap Growth [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.2		0.2
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	3.3		3.9
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.2		1.4
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.3		1.6
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.7		2.1
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.8
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.7		0.8
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.6		1.8
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.6
Other Postretirement Benefits Plans [Member] | International-Developed Markets [Member] | Equity Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.6		0.8
Other Postretirement Benefits Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.8		0.9
Other Postretirement Benefits Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.3		0.3
Other Postretirement Benefits Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.3		0.4
Other Postretirement Benefits Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.8		0.9
Other Postretirement Benefits Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.3		0.3
Other Postretirement Benefits Plans [Member] | International-Emerging Markets [Member] | Equity Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.3		0.4
Other Postretirement Benefits Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	6.1		6.3
Other Postretirement Benefits Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.6		4.5
Other Postretirement Benefits Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.1		1.4
Other Postretirement Benefits Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	6.1		6.3
Other Postretirement Benefits Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.6		4.5
Other Postretirement Benefits Plans [Member] | Corporate Bonds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.1		1.4
Other Postretirement Benefits Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	5.6		4.9
Other Postretirement Benefits Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.1		3.3
Other Postretirement Benefits Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		1.0
Other Postretirement Benefits Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	5.6		4.9
Other Postretirement Benefits Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	4.1		3.3
Other Postretirement Benefits Plans [Member] | Government And Agency Obligations [Member] | Fixed Income Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		1.0
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	25.4		24.0
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	16.5		16.0
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	2.6		2.3
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	21.6		20.1
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	15.1		14.6
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.0		0.7
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	3.8		3.9
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.4		1.4
Other Postretirement Benefits Plans [Member] | Fixed Income Funds [Member] | Fixed Income Securities [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	1.6		1.6
Other Postretirement Benefits Plans [Member] | Accrued Investment Income [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.1
Other Postretirement Benefits Plans [Member] | Accrued Investment Income [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.2
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	121.1		123.8
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	75.0		78.0
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	25.3		25.4
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 1 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	98.4		100.1
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 1 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	62.4		65.5
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 1 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	18.7		18.0
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 2 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	22.6		23.6
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 2 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	12.6		12.4
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 2 [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	6.6		7.4
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	0.1		0.1
Other Postretirement Benefits Plans [Member] | Subtotal [Member] | Level 3 [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross			0.1
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	127.4		127.6		121.6
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	97.0		95.6		92.0
Due To Brokers, Net [Member] | Defined Benefit Pension Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(4.7)	[1]	(0.6)	[1]
Due To Brokers, Net [Member] | Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(1.6)	[1]	(0.2)	[1]
Due To Brokers, Net [Member] | Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(1.5)	[1]	(0.2)	[1]
Due To Brokers, Net [Member] | Defined Benefit Pension Plans [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross				[1]
Due To Brokers, Net [Member] | Other Postretirement Benefits Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(0.2)	[1]	(0.1)	[1]
Due To Brokers, Net [Member] | Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(0.1)	[1]	(0.1)	[1]
Due To Borrowers For Securities Lending Program [Member] | Defined Benefit Pension Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(15.3)		(23.6)
Due To Borrowers For Securities Lending Program [Member] | Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(5.1)		(7.4)
Due To Borrowers For Securities Lending Program [Member] | Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(4.9)		(7.3)
Due To Borrowers For Securities Lending Program [Member] | Defined Benefit Pension Plans [Member] | Level 3 [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross
Due To Borrowers For Securities Lending Program [Member] | Other Postretirement Benefits Plans [Member] | Alliant Energy Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(0.6)		(1.1)
Due To Borrowers For Securities Lending Program [Member] | Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	(0.4)		(0.7)
Due To Borrowers For Securities Lending Program [Member] | Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, gross	$ (0.1)		$ (0.3)

[1] This category represents pending trades with brokers.