Benefit Plans (Expected Benefit Payments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	$ 78.5
Expected benefit payments, 2014	80.9
Expected benefit payments, 2015	84.8
Expected benefit payments, 2016	83.0
Expected benefit payments, 2017	85.8
Expected benefit payments, 2018 - 2022	458.3
Defined Benefit Plans Qualified And Non Qualified Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	61.9
Expected benefit payments, 2014	64.7
Expected benefit payments, 2015	68.5
Expected benefit payments, 2016	66.6
Expected benefit payments, 2017	69.2
Expected benefit payments, 2018 - 2022	374.1
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	16.6
Expected benefit payments, 2014	16.2
Expected benefit payments, 2015	16.3
Expected benefit payments, 2016	16.4
Expected benefit payments, 2017	16.6
Expected benefit payments, 2018 - 2022	84.2
IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	36.5
Expected benefit payments, 2014	37.6
Expected benefit payments, 2015	39.2
Expected benefit payments, 2016	38.1
Expected benefit payments, 2017	40.7
Expected benefit payments, 2018 - 2022	212.1
IPL [Member] | Defined Benefit Plans Qualified And Non Qualified Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	15.9
Expected benefit payments, 2014	16.9
Expected benefit payments, 2015	17.9
Expected benefit payments, 2016	18.9
Expected benefit payments, 2017	20.0
Expected benefit payments, 2018 - 2022	116.0
IPL [Member] | Directly Assigned Defined Benefit Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	12.6
Expected benefit payments, 2014	13.4
Expected benefit payments, 2015	14.1
Expected benefit payments, 2016	12.1
Expected benefit payments, 2017	13.5
Expected benefit payments, 2018 - 2022	60.2
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	8.0
Expected benefit payments, 2014	7.3
Expected benefit payments, 2015	7.2
Expected benefit payments, 2016	7.1
Expected benefit payments, 2017	7.2
Expected benefit payments, 2018 - 2022	35.9
WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	31.2
Expected benefit payments, 2014	31.9
Expected benefit payments, 2015	34.2
Expected benefit payments, 2016	33.5
Expected benefit payments, 2017	34.3
Expected benefit payments, 2018 - 2022	186.0
WPL [Member] | Defined Benefit Plans Qualified And Non Qualified Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	14.4
Expected benefit payments, 2014	15.2
Expected benefit payments, 2015	16.1
Expected benefit payments, 2016	17.0
Expected benefit payments, 2017	18.0
Expected benefit payments, 2018 - 2022	108.7
WPL [Member] | Directly Assigned Defined Benefit Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	10.5
Expected benefit payments, 2014	10.2
Expected benefit payments, 2015	11.4
Expected benefit payments, 2016	9.8
Expected benefit payments, 2017	9.5
Expected benefit payments, 2018 - 2022	43.0
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments, 2013	6.3
Expected benefit payments, 2014	6.5
Expected benefit payments, 2015	6.7
Expected benefit payments, 2016	6.7
Expected benefit payments, 2017	6.8
Expected benefit payments, 2018 - 2022	$ 34.3