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Benefit Plans Benefit Plans (Plan Assets and Percentage Associated with Entity's Benefit Plans) (Details) (Directly Assigned Defined Benefit Pension Benefits [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair values of directly assigned amounts	$ 136.5	$ 127.4	$ 127.6
Percentage represented	14.10%	14.20%
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair values of directly assigned amounts	$ 102.2	$ 97.0	$ 95.6
Percentage represented	10.60%	10.80%

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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