Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	$ 1,166.0	$ 1,255.0
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	1,166.0	1,255.0
Fair value of plan assets	895.0	1,018.1
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	1,206.3	1,301.5
Fair value of plan assets	895.0	1,018.1
Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	221.4	231.1
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	221.4	231.1
Fair value of plan assets	106.9	121.6
IPL [Member] \| Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	531.0	575.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	531.0	575.5
Fair value of plan assets	422.7	484.7
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	556.1	603.1
Fair value of plan assets	422.7	484.7
IPL [Member] \| Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	91.3	96.4
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	91.3	96.4
Fair value of plan assets	69.2	78.7
WPL [Member] \| Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	493.8	532.5
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	493.8	532.5
Fair value of plan assets	386.8	440.3
Plans with projected benefit obligations in excess of plan assets:
Projected benefit obligations	505.9	547.6
Fair value of plan assets	386.8	440.3
WPL [Member] \| Other Postretirement Benefits Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligations	89.7	94.0
Plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligations	89.7	94.0
Fair value of plan assets	$ 18.7	$ 21.8

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2417-114920

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2417-114920

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

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