Derivative Instruments (Gains And Losses From Derivative Instruments Not Designated As Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Regulatory Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	$ (25.9)	$ 7.7	$ (16.4)	$ (32.0)
Regulatory Assets [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	(6.9)	5.0	(4.2)	(17.2)
Regulatory Assets [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	(19.0)	2.7	(12.2)	(14.8)
Regulatory Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	1.2	4.1	17.6	5.5
Regulatory Liabilities [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	(2.5)	4.4	6.3	4.4
Regulatory Liabilities [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as hedging instruments	$ 3.7	$ (0.3)	$ 11.3	$ 1.1