Benefit Plans (Accumulated Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,029.4	$ 915.5
Plans with accumulated benefit obligations in excess of plan assets, Accumulated benefit obligations	1,029.4	915.5
Plans with accumulated benefit obligations in excess of plan assets, Fair value of plan assets	897.4	823.0
Plans with projected benefit obligations in excess of plan assets, Projected benefit obligations	1,081.4	953.0
Plans with projected benefit obligations in excess of plan assets, Fair value of plan assets	897.4	823.0
Defined Benefit Pension Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	314.6	274.1
Plans with accumulated benefit obligations in excess of plan assets, Accumulated benefit obligations	314.6	274.1
Plans with accumulated benefit obligations in excess of plan assets, Fair value of plan assets	298.7	256.9
Plans with projected benefit obligations in excess of plan assets, Projected benefit obligations	344.5	293.8
Plans with projected benefit obligations in excess of plan assets, Fair value of plan assets	298.7	256.9
Defined Benefit Pension Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	314.7	270.2
Plans with accumulated benefit obligations in excess of plan assets, Accumulated benefit obligations	314.7	270.2
Plans with accumulated benefit obligations in excess of plan assets, Fair value of plan assets	289.6	255.2
Plans with projected benefit obligations in excess of plan assets, Projected benefit obligations	332.5	283.3
Plans with projected benefit obligations in excess of plan assets, Fair value of plan assets	289.6	255.2
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	224.2	274.9
Plans with accumulated benefit obligations in excess of plan assets, Accumulated benefit obligations	224.2	274.9
Plans with accumulated benefit obligations in excess of plan assets, Fair value of plan assets	120.4	122.7
Other Postretirement Benefits Plans [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	97.5	128.5
Plans with accumulated benefit obligations in excess of plan assets, Accumulated benefit obligations	97.5	128.5
Plans with accumulated benefit obligations in excess of plan assets, Fair value of plan assets	74.7	77.5
Other Postretirement Benefits Plans [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	89.6	103.3
Plans with accumulated benefit obligations in excess of plan assets, Accumulated benefit obligations	89.6	103.3
Plans with accumulated benefit obligations in excess of plan assets, Fair value of plan assets	25.1	25.0
Directly Assigned Defined Benefit Pension Benefits [Member] | IPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	155.4	144.4
Directly Assigned Defined Benefit Pension Benefits [Member] | WPL [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 115.2	$ 107.4