Benefit Plans (Accumulated Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,155.5	$ 1,029.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,155.5	1,029.4
Fair value of plan assets	965.6	897.4
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,207.5	1,081.4
Fair value of plan assets	965.6	897.4
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	223.2	224.2
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	223.2	224.2
Fair value of plan assets	123.1	120.4
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	123.1	120.4
IPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	359.6	314.6
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	359.6	314.6
Fair value of plan assets	322.3	298.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	388.4	344.5
Fair value of plan assets	322.3	298.7
IPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	96.0	97.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	96.0	97.5
Fair value of plan assets	78.8	74.7
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	78.8	74.7
WPL [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	363.7	314.7
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	363.7	314.7
Fair value of plan assets	313.2	289.6
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	380.2	332.5
Fair value of plan assets	313.2	289.6
WPL [Member] | Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	89.1	89.6
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	89.1	89.6
Fair value of plan assets	22.3	25.1
Plans with projected benefit obligations in excess of plan assets:
Fair value of plan assets	$ 22.3	$ 25.1