Income Taxes (Schedule Of Components Of Income Tax) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current tax expense (benefit):
Federal	$ (29.3)	$ 58.6	$ 7.1
State	11.6	15.7	10.6
Deferred tax expense (benefit):
Federal	157.8	99.0	165.5
State	23.9	(16.8)	4.9
Production tax credits	(24.8)	(27.1)	(11.2)
Investment tax credits	(1.7)	(1.8)	(1.8)
Provision recorded as a change in uncertain tax positions, current	8.0	16.3	(84.0)
Provision recorded as a change in uncertain tax positions, deferred	(7.6)	(38.3)	59.6
Provision recorded as a change in accrued interest	(0.2)	(0.5)	(3.0)
Income tax expense (benefit)	89.4	69.2	147.7
IPL's Tax Benefit Rider [Member]
Current tax expense (benefit):
Current income tax expense (benefit)	(48.3)	(35.9)	0
IPL [Member]
Current tax expense (benefit):
Federal	(7.7)	54.5	(22.4)
State	9.1	20.0	(7.6)
Deferred tax expense (benefit):
Federal	37.4	(11.6)	100.9
State	3.2	(16.4)	(2.8)
Production tax credits	(12.5)	(12.3)	(7.7)
Investment tax credits	(0.6)	(0.6)	(0.6)
Provision recorded as a change in uncertain tax positions, current	8.1	16.6	(41.1)
Provision recorded as a change in uncertain tax positions, deferred	(8.2)	(17.6)	26.2
Provision recorded as a change in accrued interest	(0.3)	(0.3)	(2.6)
Income tax expense (benefit)	(19.8)	(3.6)	42.3
IPL [Member] | IPL's Tax Benefit Rider [Member]
Current tax expense (benefit):
Current income tax expense (benefit)	(48.3)	(35.9)	0
WPL [Member]
Current tax expense (benefit):
Federal	7.2	(4.3)	26.8
State	(0.9)	(7.1)	14.7
Deferred tax expense (benefit):
Federal	81.1	111.3	63.3
State	20.3	19.0	6.6
Production tax credits	(12.3)	(14.8)	(3.5)
Investment tax credits	(1.1)	(1.2)	(1.2)
Provision recorded as a change in uncertain tax positions, current	(0.1)	(0.3)	(41.7)
Provision recorded as a change in uncertain tax positions, deferred	0.6	(20.7)	33.3
Provision recorded as a change in accrued interest	(0.2)	0	0
Income tax expense (benefit)	94.6	81.9	98.3
WPL [Member] | IPL's Tax Benefit Rider [Member]
Current tax expense (benefit):
Current income tax expense (benefit)	$ 0	$ 0	$ 0