Derivative Instruments (Gains And Losses From Derivative Instruments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Regulatory Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	$ (21.2)	$ (4.6)	$ (42.1)	$ 5.5
Regulatory Assets [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	(12.1)	(4.1)	(23.8)	1.4
Regulatory Assets [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	(9.1)	(0.5)	(18.3)	4.1
Regulatory Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	(4.6)	21.3	(2.4)	69.4
Regulatory Liabilities [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	3.6	3.8	2.2	15.9
Regulatory Liabilities [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	$ (8.2)	$ 17.5	$ (4.6)	$ 53.5