Derivative Instruments (Gains And Losses From Derivative Instruments Not Designated As Hedging Instruments) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
IPL [Member] | Commodity Contracts [Member] | Regulatory Assets [Member]
Gain (loss) on derivative instruments	$ (3.0)	$ 6.3	$ (5.4)	$ (25.8)
WPL [Member] | Commodity Contracts [Member] | Regulatory Assets [Member]
Gain (loss) on derivative instruments	(2.3)	7.5	(3.9)	(13.4)
Commodity Contracts [Member] | Regulatory Assets [Member]
Gain (loss) on derivative instruments	(5.3)	13.8	(9.3)	(39.2)
IPL [Member] | Commodity Contracts [Member] | Regulatory Liabilities [Member]
Gain (loss) on derivative instruments	2.4	(0.5)	3.2	7.3
IPL [Member] | Regulatory Liabilities [Member] | Foreign Contracts [Member]
Gain (loss) on derivative instruments				3.8
WPL [Member] | Commodity Contracts [Member] | Regulatory Liabilities [Member]
Gain (loss) on derivative instruments	1.0	(1.0)	1.7	(1.3)
Commodity Contracts [Member] | Regulatory Liabilities [Member]
Gain (loss) on derivative instruments	3.4	(1.5)	4.9	6.0
Regulatory Liabilities [Member] | Foreign Contracts [Member]
Gain (loss) on derivative instruments				$ 3.8