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Postretirement Health Care and Life Insurance Benefits Net Periodic Postretirement Benefit Cost (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2016
Successor |  |  |  |
Components of net periodic postretirement benefit cost |  |  |  |
Net periodic postretirement benefit cost |  | $ 21.9 | $ 18.1 |
Successor | Postretirement Health Care and Life Insurance Benefits |  |  |  |
Components of net periodic postretirement benefit cost |  |  |  |
Service cost for benefits earned |  | 6.9 | 8.2 |
Interest cost on accumulated postretirement benefit obligation |  | 24.2 | 28.3 |
Amortization of prior service credit |  | 0.0 | 0.0 |
Amortization of actuarial loss |  | 0.0 | 0.0 |
Net actuarial gain |  | (22.0) | (128.4) |
Net periodic postretirement benefit cost |  | $ 9.1 | $ (91.9) |
Predecessor |  |  |  |
Components of net periodic postretirement benefit cost |  |  |  |
Net periodic postretirement benefit cost | $ 14.4 |  |  | $ 57.1
Predecessor | Postretirement Health Care and Life Insurance Benefits |  |  |  |
Components of net periodic postretirement benefit cost |  |  |  |
Service cost for benefits earned | 2.3 |  |  | 10.4
Interest cost on accumulated postretirement benefit obligation | 8.4 |  |  | 34.5
Amortization of prior service credit | (2.3) |  |  | (9.2)
Amortization of actuarial loss | 5.5 |  |  | 20.4
Net actuarial gain | 0.0 |  |  | 0.0
Net periodic postretirement benefit cost | $ 13.9 |  |  | $ 56.1

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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 gain (loss) recognized in net periodic benefit (cost) credit of 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of 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 (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to 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 (h)(2) -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 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of net periodic benefit cost (credit) 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 (h) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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