Derivative Instruments (Fair Value Of Financial Instruments) (Details) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Current derivative assets	$ 17.5	$ 23.5
Non-current derivative assets	6.1	2.7
Current derivative liabilities	13.5	31.1
Non-current derivative liabilities	3.6	9.3
IPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	10.9	17.0
Non-current derivative assets	1.6	0.5
Current derivative liabilities	5.7	14.1
Non-current derivative liabilities	0.4	2.0
WPL [Member]
Derivatives, Fair Value [Line Items]
Current derivative assets	6.6	6.5
Non-current derivative assets	4.5	2.2
Current derivative liabilities	7.8	17.0
Non-current derivative liabilities	$ 3.2	$ 7.3