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Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | $ 1,269.0 | $ 1,201.5
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 1,269.0 | 1,201.5
Fair value of plan assets | 950.7 | 895.7
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 1,303.1 | 1,244.3
Fair value of plan assets | 950.7 | 895.7
Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 222.3 | 220.1
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 222.3 | 220.1
Fair value of plan assets | 111.1 | 105.8
Plans with projected benefit obligations in excess of plan assets: |  |
Fair value of plan assets | 107.5 | 102.8
IPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 573.1 | 546.7
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 573.1 | 546.7
Fair value of plan assets | 443.7 | 422.0
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 592.9 | 570.4
Fair value of plan assets | 443.7 | 422.0
IPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 89.4 | 90.1
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 89.4 | 90.1
Fair value of plan assets | 72.9 | 68.2
Plans with projected benefit obligations in excess of plan assets: |  |
Fair value of plan assets |  | 68.2
WPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 548.1 | 513.2
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 548.1 | 513.2
Fair value of plan assets | 415.0 | 389.7
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 559.8 | 529.2
Fair value of plan assets | 415.0 | 389.7
WPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 90.4 | 88.9
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 90.4 | 88.9
Fair value of plan assets | $ 18.7 | $ 18.6

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- Definition 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. + References 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=65897934&loc=d3e1928-114920 + Details
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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=73719040&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=65897934&loc=d3e2417-114920 + Details
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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. + 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=73719040&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=65897934&loc=d3e2417-114920 + Details
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