Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax liabilities, gross	$ 2,507.0	$ 2,320.4
Deferred tax assets, valuation allowance	1.9	1.2
Deferred tax assets, net of valuation allowance	743.0	751.0
Deferred tax liabilities, net of deferred tax assets and valuation allowance	1,764.0	1,569.4
Other current assets	(170.2)	(22.8)
Deferred income taxes	1,934.2	1,592.2
IPL [Member]
Deferred tax assets, gross	(383.4)	(396.5)
Deferred tax liabilities, gross	1,391.4	1,319.9
Deferred tax assets and liabilities, net	1,008.0	923.4
Deferred tax liabilities, net of deferred tax assets and valuation allowance	1,008.0	923.4
Other current assets	(79.3)	(13.5)
Deferred income taxes	1,087.3	936.9
WPL [Member]
Deferred tax assets, gross	(222.8)	(208.1)
Deferred tax liabilities, gross	981.3	874.6
Deferred tax assets and liabilities, net	758.5	666.5
Deferred tax liabilities, net of deferred tax assets and valuation allowance	758.5	666.5
Other current assets	(85.6)	(6.0)
Deferred income taxes	844.1	672.5
Property [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	2,143.8	1,926.4
Deferred tax assets and liabilities, net	2,143.8	1,926.4
Property [Member] | IPL [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	1,243.9	1,134.1
Deferred tax assets and liabilities, net	1,243.9	1,134.1
Property [Member] | WPL [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	793.3	697.1
Deferred tax assets and liabilities, net	793.3	697.1
Investment In ATC [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	104.3	93.8
Deferred tax assets and liabilities, net	104.3	93.8
Investment In ATC [Member] | WPL [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	104.3	93.8
Deferred tax assets and liabilities, net	104.3	93.8
Pension And Other Postretirement Benefits Obligations [Member] | IPL [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	64.9	67.6
Deferred tax assets and liabilities, net	64.9	67.6
Pension And Other Postretirement Benefits Obligations [Member] | WPL [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	46.4	49.6
Deferred tax assets and liabilities, net	46.4	49.6
Deferred Portion of Tax Gain On IPL's Electric Transmission Assets Sale [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	50.7	75.5
Deferred tax assets and liabilities, net	50.7	75.5
Deferred Portion of Tax Gain On IPL's Electric Transmission Assets Sale [Member] | IPL [Member]
Deferred tax assets, gross	0	0
Deferred tax liabilities, gross	50.7	75.5
Deferred tax assets and liabilities, net	50.7	75.5
Net Operating Losses Carryforward - State [Member]
Deferred tax assets, gross	(46.8)	(39.9)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(46.8)	(39.9)
Customer Advances [Member] | WPL [Member]
Deferred tax assets, gross	(9.6)	(11.6)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(9.6)	(11.6)
Federal Credit Carryforward [Member]
Deferred tax assets, gross	(133.8)	(107.4)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(133.8)	(107.4)
Federal Credit Carryforward [Member] | IPL [Member]
Deferred tax assets, gross	(37.4)	(25.0)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(37.4)	(25.0)
Federal Credit Carryforward [Member] | WPL [Member]
Deferred tax assets, gross	(39.4)	(26.1)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(39.4)	(26.1)
Regulatory Liability, IPL's Tax Benefit Rider [Member]
Deferred tax assets, gross	(144.6)	(140.6)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(144.6)	(140.6)
Regulatory Liability, IPL's Tax Benefit Rider [Member] | IPL [Member]
Deferred tax assets, gross	(144.6)	(140.6)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(144.6)	(140.6)
Net Operating Losses Carryforward - Federal [Member]
Deferred tax assets, gross	(306.0)	(336.1)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(306.0)	(336.1)
Net Operating Losses Carryforward - Federal [Member] | IPL [Member]
Deferred tax assets, gross	(131.0)	(154.4)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(131.0)	(154.4)
Net Operating Losses Carryforward - Federal [Member] | WPL [Member]
Deferred tax assets, gross	(142.2)	(141.1)
Deferred tax liabilities, gross	0	0
Deferred tax assets and liabilities, net	(142.2)	(141.1)
Other [Member]
Deferred tax assets, gross	(113.7)	(128.2)
Deferred tax liabilities, gross	208.2	224.7
Deferred tax assets and liabilities, net	94.5	96.5
Other [Member] | IPL [Member]
Deferred tax assets, gross	(70.4)	(76.5)
Deferred tax liabilities, gross	31.9	42.7
Deferred tax assets and liabilities, net	(38.5)	(33.8)
Other [Member] | WPL [Member]
Deferred tax assets, gross	(31.6)	(29.3)
Deferred tax liabilities, gross	37.3	34.1
Deferred tax assets and liabilities, net	5.7	4.8
Subtotal [Member]
Deferred tax assets, gross	(744.9)	(752.2)
Deferred tax liabilities, gross	2,507.0	2,320.4
Deferred tax assets and liabilities, net	$ 1,762.1	$ 1,568.2