Derivative Instruments (Fair Value Of Financial Instruments) (Details) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 34.7	$ 23.5
Non-current derivative assets	2.1	2.7
Current derivative liabilities	11.8	31.1
Non-current derivative liabilities	15.3	9.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	29.9	17.0
Non-current derivative assets	1.6	0.5
Current derivative liabilities	4.9	14.1
Non-current derivative liabilities	1.8	2.0
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	4.8	6.5
Non-current derivative assets	0.5	2.2
Current derivative liabilities	6.9	17.0
Non-current derivative liabilities	$ 13.5	$ 7.3