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Pension and Postretirement Benefit Costs (Details) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) | Mar. 31, 2017 USD ($) | Apr. 02, 2017 plan
Successor |  |  |
Components of net periodic benefit cost |  |  |
Net periodic pension (benefit) cost | $ 4.5 |  |
Successor | Defined benefit pension plans |  |  |
Components of net periodic benefit cost |  |  |
Service cost for benefits earned | 0.6 |  |
Interest cost on projected benefit obligation | 7.8 |  |
Expected return on plan assets | (10.7) |  |
Amortization of prior service cost and net actuarial loss | 0.0 |  |
Net periodic pension (benefit) cost | $ (2.3) |  |
Other Defined Benefit Plan Information [Abstract] |  |  |
Funding threshold | 80.00% |  |
Number of non-qualified pension plans prior to reorganization | plan |  |  | 2
Defined Contribution Plan, Employer Discretionary Contribution Amount | $ 30.0 |  |
Successor | Postretirement benefit plans |  |  |
Components of net periodic benefit cost |  |  |
Service cost for benefits earned | 2.0 |  |
Interest cost on projected benefit obligation | 7.1 |  |
Amortization of prior service cost and net actuarial loss | 0.0 |  |
Net periodic pension (benefit) cost | $ 9.1 |  |
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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