Benefit Plans (Expected Benefit Payments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
2012	$ 76.8
2013	77.2
2014	80.7
2015	83.4
2016	82.5
2017 - 2021	444.1
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	36.5
2013	36.3
2014	38.0
2015	38.6
2016	37.6
2017 - 2021	205.6
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	28.8
2013	30.2
2014	31.6
2015	32.5
2016	33.5
2017 - 2021	181.6
Qualified And Non-Qualified Defined Benefit Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	58.8
2013	59.1
2014	62.3
2015	66.3
2016	65.2
2017 - 2021	355.4
Qualified Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	15.5
2013	16.2
2014	17.1
2015	18.2
2016	19.2
2017 - 2021	111.6
Qualified Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	13.0
2013	13.8
2014	14.9
2015	15.9
2016	17.0
2017 - 2021	103.3
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	12.1
2013	11.4
2014	12.3
2015	13.0
2016	11.0
2017 - 2021	57.0
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	9.0
2013	9.4
2014	9.5
2015	9.5
2016	9.2
2017 - 2021	40.9
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	18.0
2013	18.1
2014	18.4
2015	17.1
2016	17.3
2017 - 2021	88.7
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	8.9
2013	8.7
2014	8.6
2015	7.4
2016	7.4
2017 - 2021	37.0
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
2012	6.8
2013	7.0
2014	7.2
2015	7.1
2016	7.3
2017 - 2021	$ 37.4