Benefit Plans (Plan Assets And Plan Assets Percentage Associated With Entity's Benefit Plans) (Details) (Directly Assigned Defined Benefit Pension Benefits [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair values of directly assigned amounts	$ 127.4	$ 127.6
Percentage represented	14.20%	15.50%
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair values of directly assigned amounts	$ 97.0	$ 95.6
Percentage represented	10.80%	11.60%