Derivative Instruments (Gains And Losses From Derivative Instruments Not Designated As Hedging Instruments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commodity Contracts [Member] | Regulatory Assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	$ (37.9)	$ (79.6)	$ (78.4)
Commodity Contracts [Member] | Regulatory Assets [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	(16.8)	(42.4)	(47.8)
Commodity Contracts [Member] | Regulatory Assets [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	(21.1)	(37.2)	(30.6)
Commodity Contracts [Member] | Regulatory Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	20.3	9.3	11.5
Commodity Contracts [Member] | Regulatory Liabilities [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	13.5	6.4	10.6
Commodity Contracts [Member] | Regulatory Liabilities [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	6.8	2.9	0.9
Foreign Contracts [Member] | Regulatory Liabilities [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	0	0	3.8
Foreign Contracts [Member] | Regulatory Liabilities [Member] | IPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	0	0	3.8
Foreign Contracts [Member] | Regulatory Liabilities [Member] | WPL [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivative instruments not designated as heding instruments	$ 0	$ 0	$ 0