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Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Contribution Benefit Plans |  |  |
Company contribution to 401(k) savings plan for Wisconsin Energy Corporation management employees hired after December 31, 2014 (percentage) | 6.00% |  |
Pension Costs |  |  |
Change in benefit obligation |  |  |
Obligation at January 1 | $ 3,058.8 | $ 3,083.0 |
Service cost | 44.6 | 45.4 | $ 30.4
Interest cost | 121.8 | 130.8 | 94.3
Participant contributions | 0.0 | 0.0 |
Plan amendments | 0.0 | (3.0) |
Actuarial loss (gain) | 162.6 | 71.7 |
Benefit payments | (224.1) | (269.1) |
Obligation at December 31 | 3,163.7 | 3,058.8 | 3,083.0
Change in fair value of plan assets |  |  |
Beginning balance at January 1 | 2,709.2 | 2,755.1 |
Actual return on plan assets | 368.7 | 199.4 |
Employer contributions | 113.0 | 23.8 |
Participant contributions | 0.0 | 0.0 |
Benefit payments | (224.1) | (269.1) |
Ending balance at December 31 | 2,966.8 | 2,709.2 | 2,755.1
Funded status at December 31 | (196.9) | (349.6) |
OPEB Costs |  |  |
Change in benefit obligation |  |  |
Obligation at January 1 | 818.4 | 842.0 |
Service cost | 24.1 | 26.1 | 20.7
Interest cost | 32.9 | 37.0 | 26.7
Participant contributions | 13.4 | 16.4 |
Plan amendments | (36.4) | (18.9) |
Actuarial loss (gain) | 12.9 | (36.5) |
Benefit payments | (48.8) | (49.1) |
Federal subsidy on benefits paid | 2.0 | 1.4 |
Obligation at December 31 | 818.5 | 818.4 | 842.0
Change in fair value of plan assets |  |  |
Beginning balance at January 1 | 773.5 | 749.8 |
Actual return on plan assets | 95.9 | 51.5 |
Employer contributions | 7.5 | 4.9 |
Participant contributions | 13.4 | 16.4 |
Benefit payments | (48.8) | (49.1) |
Ending balance at December 31 | 841.5 | 773.5 | $ 749.8
Funded status at December 31 | $ 23.0 | $ (44.9) |

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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, before benefit payment, of receipt from prescription drug subsidy which increases benefit obligation of defined benefit postretirement plan. + 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 + 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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- Definition Percentage company contributes to the 401(k) savings plan for Wisconsin Energy Corporation's management employees hired after December 31, 2014. + References No definition available. + Details
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