Derivative Instruments (Fair Value Of Financial Instruments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Current derivative liabilities	$ 61.9	$ 55.9
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative liabilities	30.1	24.5
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative liabilities	31.8	31.4
Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	5.6	12.7
Non-current derivative assets	4.8	3.0
Current derivative liabilities	61.9	55.9
Non-current derivative liabilities	21.3	22.1
Commodity Contracts [Member] | IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	3.9	9.2
Non-current derivative assets	2.0	1.4
Current derivative liabilities	30.1	24.5
Non-current derivative liabilities	8.0	9.1
Commodity Contracts [Member] | WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	1.7	3.5
Non-current derivative assets	2.8	1.6
Current derivative liabilities	31.8	31.4
Non-current derivative liabilities	$ 13.3	$ 13.0