Benefit Plans (Accumulated Benefit Obligations) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,255.0	$ 1,071.7
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,255.0	406.5
Fair value of plan assets	1,018.1	347.6
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,301.5	1,113.4
Fair value of plan assets	1,018.1	1,022.9
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	231.1	208.7
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	231.1	208.7
Fair value of plan assets	121.6	124.9
IPL [Member] \| Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	575.5	491.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	575.5	159.3
Fair value of plan assets	484.7	144.6
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	603.1	514.0
Fair value of plan assets	484.7	485.9
IPL [Member] \| Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	96.4	87.8
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	96.4	87.8
Fair value of plan assets	78.7	81.2
WPL [Member] \| Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	532.5	446.7
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	532.5	115.6
Fair value of plan assets	440.3	106.8
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	547.6	460.8
Fair value of plan assets	440.3	438.8
WPL [Member] \| Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	94.0	85.6
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	94.0	85.6
Fair value of plan assets	$ 21.8	$ 21.7

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Accumulated Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6503844

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 55

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=34716465&loc=d3e4179-114921

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

+ References

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-Publisher FASB

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-Paragraph 3

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