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

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

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If aggregate disclosures are presented, the aggregate pension accumulated benefit obligation for pension plans with accumulated benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented.

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- Definition

If aggregate disclosures are presented, the aggregate fair value of plan assets for pension plans with accumulated benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented.

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