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Pension and Postretirement Benefit Costs (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Apr. 01, 2017 | Sep. 30, 2017 | Sep. 30, 2018
Successor |  |  |  |  |
Components of net periodic benefit cost |  |  |  |  |
Net periodic pension (benefit) cost | $ 4.5 | $ 6.6 |  | $ 13.2 | $ 13.6
Successor | Defined benefit pension plans |  |  |  |  |
Components of net periodic benefit cost |  |  |  |  |
Service cost for benefits earned | 0.6 | 0.5 |  | 1.1 | 1.7
Interest cost on projected benefit obligation | 7.9 | 9.4 |  | 18.7 | 23.6
Expected return on plan assets | (10.7) | (11.2) |  | (22.4) | (32.1)
Amortization of prior service cost and net actuarial loss | 0.0 | 0.0 |  | 0.0 | 0.0
Net periodic pension (benefit) cost | $ (2.2) | (1.3) |  | (2.6) | $ (6.8)
Other Defined Benefit Plan Information [Abstract] |  |  |  |  |
Funding threshold | 80.00% |  |  |  | 80.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount | $ 20.0 |  |  |  | $ 62.0
Successor | Postretirement benefit plans |  |  |  |  |
Components of net periodic benefit cost |  |  |  |  |
Service cost for benefits earned | 2.1 | 2.3 |  | 4.6 | 6.2
Interest cost on projected benefit obligation | 7.0 | 8.2 |  | 16.5 | 21.2
Amortization of prior service cost and net actuarial loss | 0.0 | 0.0 |  | 0.0 | 0.0
Net periodic pension (benefit) cost | $ 9.1 | $ 10.5 |  | $ 21.1 | $ 27.4
Predecessor |  |  |  |  |
Components of net periodic benefit cost |  |  |  |  |
Net periodic pension (benefit) cost |  |  | $ 14.4 |  |
Predecessor | Defined benefit pension plans |  |  |  |  |
Components of net periodic benefit cost |  |  |  |  |
Service cost for benefits earned |  |  | 0.6 |  |
Interest cost on projected benefit obligation |  |  | 9.7 |  |
Expected return on plan assets |  |  | (11.0) |  |
Amortization of prior service cost and net actuarial loss |  |  | 6.4 |  |
Net periodic pension (benefit) cost |  |  | 5.7 |  |
Predecessor | Postretirement benefit plans |  |  |  |  |
Components of net periodic benefit cost |  |  |  |  |
Service cost for benefits earned |  |  | 2.3 |  |
Interest cost on projected benefit obligation |  |  | 8.4 |  |
Amortization of prior service cost and net actuarial loss |  |  | 3.2 |  |
Net periodic pension (benefit) cost |  |  | $ 13.9 |  |

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- Definition Percentage of company's qualified pension plans on a GAAP accounting basis. + References No definition available. + 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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