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Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Pension Plans, Defined Benefit [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | $ 1,303.1 | $ 1,244.3 |
Service cost | 12.1 | 12.5 | $ 12.6
Interest cost | 46.8 | 51.0 | 53.0
Plan participants' contributions | 0.0 | 0.0 |
Actuarial (gain) loss | (96.2) | 83.6 |
Gross benefits paid | (90.8) | (88.3) |
Net benefit obligation at December 31 | 1,175.0 | 1,303.1 | 1,244.3
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 950.7 | 895.7 |
Actual return on plan assets | (57.8) | 136.7 |
Employer contributions | 6.5 | 6.6 |
Plan participants' contributions | 0.0 | 0.0 |
Gross benefits paid | (90.8) | (88.3) |
Fair value of plan assets at December 31 | 808.6 | 950.7 | 895.7
Under funded status at December 31 | (366.4) | (352.4) |
Pension Plans, Defined Benefit [Member] | IPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 592.9 | 570.4 |
Service cost | 7.4 | 7.3 | 7.5
Interest cost | 21.4 | 23.5 | 24.5
Plan participants' contributions | 0.0 | 0.0 |
Actuarial (gain) loss | (44.4) | 34.9 |
Gross benefits paid | (43.4) | (43.2) |
Net benefit obligation at December 31 | 533.9 | 592.9 | 570.4
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 443.7 | 422.0 |
Actual return on plan assets | (26.8) | 64.3 |
Employer contributions | 4.3 | 0.6 |
Plan participants' contributions | 0.0 | 0.0 |
Gross benefits paid | (43.4) | (43.2) |
Fair value of plan assets at December 31 | 377.8 | 443.7 | 422.0
Under funded status at December 31 | (156.1) | (149.2) |
Pension Plans, Defined Benefit [Member] | WPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 559.8 | 529.2 |
Service cost | 4.4 | 4.9 | 4.9
Interest cost | 20.2 | 21.8 | 22.3
Plan participants' contributions | 0.0 | 0.0 |
Actuarial (gain) loss | (40.0) | 38.3 |
Gross benefits paid | (37.8) | (34.4) |
Net benefit obligation at December 31 | 506.6 | 559.8 | 529.2
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 415.0 | 389.7 |
Actual return on plan assets | (25.1) | 59.6 |
Employer contributions | 0.1 | 0.1 |
Plan participants' contributions | 0.0 | 0.0 |
Gross benefits paid | (37.8) | (34.4) |
Fair value of plan assets at December 31 | 352.2 | 415.0 | 389.7
Under funded status at December 31 | (154.4) | (144.8) |
Other Postretirement Benefits Plans [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 222.3 | 220.1 |
Service cost | 4.2 | 5.0 | 5.3
Interest cost | 7.7 | 8.6 | 9.4
Plan participants' contributions | 3.1 | 2.9 |
Actuarial (gain) loss | (8.2) | 5.4 |
Gross benefits paid | (23.0) | (19.7) |
Net benefit obligation at December 31 | 206.1 | 222.3 | 220.1
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 111.1 | 105.8 |
Actual return on plan assets | (2.6) | 12.9 |
Employer contributions | 10.5 | 9.2 |
Plan participants' contributions | 3.1 | 2.9 |
Gross benefits paid | (23.0) | (19.7) |
Fair value of plan assets at December 31 | 99.1 | 111.1 | 105.8
Under funded status at December 31 | (107.0) | (111.2) |
Other Postretirement Benefits Plans [Member] | IPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 89.4 | 90.1 |
Service cost | 1.7 | 2.1 | 2.3
Interest cost | 3.1 | 3.5 | 3.8
Plan participants' contributions | 1.0 | 1.0 |
Actuarial (gain) loss | (4.3) | (0.1) |
Gross benefits paid | (8.4) | (7.2) |
Net benefit obligation at December 31 | 82.5 | 89.4 | 90.1
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 72.9 | 68.2 |
Actual return on plan assets | (1.4) | 8.9 |
Employer contributions | 2.6 | 2.0 |
Plan participants' contributions | 1.0 | 1.0 |
Gross benefits paid | (8.4) | (7.2) |
Fair value of plan assets at December 31 | 66.7 | 72.9 | 68.2
Under funded status at December 31 | (15.8) | (16.5) |
Other Postretirement Benefits Plans [Member] | WPL [Member] |  |  |
Change in benefit obligation: |  |  |
Net benefit obligation at January 1 | 90.4 | 88.9 |
Service cost | 1.6 | 1.9 | 2.0
Interest cost | 3.1 | 3.4 | 3.8
Plan participants' contributions | 1.4 | 1.4 |
Actuarial (gain) loss | (2.5) | 4.1 |
Gross benefits paid | (10.8) | (9.3) |
Net benefit obligation at December 31 | 83.2 | 90.4 | 88.9
Change in plan assets: |  |  |
Fair value of plan assets at January 1 | 18.7 | 18.6 |
Actual return on plan assets | (0.1) | 1.2 |
Employer contributions | 7.5 | 6.8 |
Plan participants' contributions | 1.4 | 1.4 |
Gross benefits paid | (10.8) | (9.3) |
Fair value of plan assets at December 31 | 16.7 | 18.7 | $ 18.6
Under funded status at December 31 | $ (66.5) | $ (71.7) |

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + 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 (b)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + 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 (a)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan. + 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 (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contributions received by defined benefit plan from participant which increase benefit obligation. + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases 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 1 -Subparagraph (d)(5)(ii) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status. + 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 (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919 + Details
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