Derivative Instruments (Gains And Losses From Derivative Instruments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Regulatory Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	$ (27.4)	$ 8.3	$ (69.5)	$ 13.8
Regulatory Assets [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	(9.5)	7.3	(33.3)	8.7
Regulatory Assets [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	(17.9)	1.0	(36.2)	5.1
Regulatory Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	(2.5)	(6.2)	(4.9)	63.2
Regulatory Liabilities [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	(1.2)	(2.0)	1.0	13.9
Regulatory Liabilities [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from commodity derivative instruments	$ (1.3)	$ (4.2)	$ (5.9)	$ 49.3