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Benefit Plans (Accumulated Benefit Obligations) (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | $ 1,139.9 | $ 1,269.0
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 1,139.9 | 1,269.0
Fair value of plan assets | 808.6 | 950.7
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 1,175.0 | 1,303.1
Fair value of plan assets | 808.6 | 950.7
Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 206.1 | 222.3
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 206.1 | 222.3
Fair value of plan assets | 99.1 | 111.1
Plans with projected benefit obligations in excess of plan assets: |  |
Fair value of plan assets | 76.8 | 107.5
IPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 514.3 | 573.1
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 514.3 | 573.1
Fair value of plan assets | 377.8 | 443.7
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 533.9 | 592.9
Fair value of plan assets | 377.8 | 443.7
IPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 82.5 | 89.4
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 82.5 | 89.4
Fair value of plan assets | 66.7 | 72.9
Plans with projected benefit obligations in excess of plan assets: |  |
Fair value of plan assets | 47.7 | 72.9
WPL [Member] | Pension Plans, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 494.8 | 548.1
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 494.8 | 548.1
Fair value of plan assets | 352.2 | 415.0
Plans with projected benefit obligations in excess of plan assets: |  |
Projected benefit obligations | 506.6 | 559.8
Fair value of plan assets | 352.2 | 415.0
WPL [Member] | Other Postretirement Benefits Plans [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligations | 83.2 | 90.4
Plans with accumulated benefit obligations in excess of plan assets: |  |
Accumulated benefit obligations | 83.2 | 90.4
Fair value of plan assets | $ 16.7 | $ 18.7

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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: 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=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of accumulated benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + 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 (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Fair value of plan assets for defined benefit pension plan with accumulated benefit obligation in excess of plan assets. + 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 (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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