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Pension and Savings Plans (Tables) - Pension Plans, Defined Benefit [Member] | 12 Months Ended
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Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |
Net periodic pension cost | Net periodic pension (benefit) cost included the following components:
| Successor | Predecessor
| April 2 through December 31, 2017 | January 1 through April 1, 2017 |  | Year Ended December 31, 2016 |  | Year Ended December 31, 2015
| (Dollars in millions)
Service cost for benefits earned | $ | 1.6 |  | $ | 0.6 |  |  | $ | 2.5 |  |  | $ | 2.7 |
Interest cost on projected benefit obligation | 28.0 |  | 9.7 |  |  | 41.5 |  |  | 40.4 |
Expected return on plan assets | (33.5 | ) | (11.0 | ) |  | (45.3 | ) |  | (48.2 | )
Amortization of prior service cost | — |  | 0.1 |  |  | 0.3 |  |  | 1.0 |
Amortization of net actuarial losses | — |  | 6.3 |  |  | 24.7 |  |  | 39.6 |
Settlement charge | 2.1 |  | — |  |  | — |  |  | — |
Net actuarial gain | (23.5 | ) | — |  |  | — |  |  | — |
Net periodic pension (benefit) cost | $ | (25.3 | ) | $ | 5.7 |  |  | $ | 23.7 |  |  | $ | 35.5 |
Amounts recognized in accumulated other comprehensive loss | The following includes pre-tax amounts recorded in “Accumulated other comprehensive income (loss)”:
| Predecessor
| January 1 through April 1, 2017 |  | Year Ended December 31, 2016 |  | Year Ended December 31, 2015
| (Dollars in millions)
Net actuarial loss arising during period | $ | — |  |  | $ | 6.6 |  |  | $ | 30.6 |
Amortization: |  |  |  |  |  |  |  |
Net actuarial loss | (6.3 | ) |  | (24.7 | ) |  | (39.6 | )
Prior service cost | (0.1 | ) |  | (0.3 | ) |  | (1.0 | )
Total recorded in “Accumulated other comprehensive income (loss)” | $ | (6.4 | ) |  | $ | (18.4 | ) |  | $ | (10.0 | )
Summary of change in benefit obligation, change in plan assets and funded status | The following summarizes the change in benefit obligation, change in plan assets and funded status of the Pension Plans:
| Successor | Predecessor
| April 2 through December 31, 2017 | January 1 through April 1, 2017 |  | Year Ended December 31, 2016
| (Dollars in millions)
Change in benefit obligation: |  |  |  |  |  |
Projected benefit obligation at beginning of period | $ | 936.4 |  | $ | 959.3 |  |  | $ | 939.3 |
Service cost | 1.6 |  | 0.6 |  |  | 2.5 |
Interest cost | 28.0 |  | 9.7 |  |  | 41.5 |
Benefits paid | (45.3 | ) | (15.0 | ) |  | (61.1 | )
Actuarial loss | 25.3 |  | — |  |  | 37.1 |
Settlement | (71.4 | ) | — |  |  | — |
Fresh start reporting adjustments | — |  | (18.2 | ) |  | — |
Projected benefit obligation at end of period | 874.6 |  | 936.4 |  |  | 959.3 |
Change in plan assets: |  |  |  |  |  |  |
Fair value of plan assets at beginning of period | 783.1 |  | 773.0 |  |  | 757.3 |
Actual return on plan assets | 80.1 |  | 25.1 |  |  | 75.7 |
Employer contributions | 30.1 |  | — |  |  | 1.1 |
Benefits paid | (45.3 | ) | (15.0 | ) |  | (61.1 | )
Settlement | (71.4 | ) | — |  |  | — |
Fair value of plan assets at end of period | 776.6 |  | 783.1 |  |  | 773.0 |
Funded status at end of period | $ | (98.0 | ) | $ | (153.3 | ) |  | $ | (186.3 | )
Amounts recognized in the consolidated balance sheets: |  |  |  |  |  |  |
Noncurrent obligation (included in “Other noncurrent liabilities”) | $ | (98.0 | ) | $ | (153.3 | ) |  | $ | (163.5 | )
Liabilities subject to compromise | — |  | — |  |  | (22.8 | )
Net amount recognized | $ | (98.0 | ) | $ | (153.3 | ) |  | $ | (186.3 | )
Weighted-average assumptions used to determine benefit obligations | The weighted-average assumptions used to determine the benefit obligations as of the end of each year were as follows:
| Successor |  |  | Predecessor
| December 31,
| 2017 |  |  | 2016
Discount rate | 3.70 | % |  |  | 4.15 | %
Measurement date | December 31, 2017 |  |  |  | December 31, 2016 |
Weighted-average assumptions used to determine net periodic benefit cost | The weighted-average assumptions used to determine net periodic pension (benefit) cost during each period were as follows:
| Successor |  |  | Predecessor
| April 2 through December 31, 2017 |  |  | January 1 through April 1, 2017 | Year Ended December 31, 2016 | Year Ended December 31, 2015
Discount rate | 4.10 | % |  |  | 4.15 | % | 4.55 | % | 4.15 | %
Expected long-term return on plan assets | 5.90 | % |  |  | 5.90 | % | 6.00 | % | 6.25 | %
Measurement date | April 1, 2017 |  |  |  | December 31, 2016 |  | December 31, 2015 |  | December 31, 2014 |
Fair value of assets in the Master Trust by category and by fair value valuation hierarchy | The following tables present the fair value of assets in the Master Trust by asset category and by fair value hierarchy:
| Successor
| December 31, 2017
| Level 1 |  | Level 2 |  | Level 3 |  | Total
| (Dollars in millions)
Mutual funds | $ | 108.0 |  |  | $ | — |  |  | $ | — |  |  | $ | 108.0 |
Corporate bonds | — |  |  | 291.1 |  |  | — |  |  | 291.1 |
U.S. government securities | 7.5 |  |  | 21.8 |  |  | — |  |  | 29.3 |
International government securities | — |  |  | 17.7 |  |  | — |  |  | 17.7 |
Cash funds | 30.8 |  |  | — |  |  | — |  |  | 30.8 |
Real estate investment trusts | — |  |  | — |  |  | 11.8 |  |  | 11.8 |
Total assets at fair value | $ | 146.3 |  |  | $ | 330.6 |  |  | $ | 11.8 |  |  | 488.7 |
Assets measured at net asset value practical expedient (1) |  |  |  |  |  |  |
Private mutual funds |  |  |  |  |  |  | 180.4 |
Common collective trusts |  |  |  |  |  |  | 107.5 |
|  |  |  |  |  |  | 287.9 |
Total plan assets |  |  |  |  |  |  | $ | 776.6 |
| Predecessor
| December 31, 2016
| Level 1 |  | Level 2 |  | Level 3 |  | Total
| (Dollars in millions)
Mutual funds | $ | 119.9 |  |  | $ | — |  |  | $ | — |  |  | $ | 119.9 |
Corporate bonds | — |  |  | 265.7 |  |  | — |  |  | 265.7 |
U.S. government securities | 25.1 |  |  | 22.7 |  |  | — |  |  | 47.8 |
International government securities | — |  |  | 12.6 |  |  | — |  |  | 12.6 |
Cash funds | 17.8 |  |  | — |  |  | — |  |  | 17.8 |
Real estate investment trusts | — |  |  | — |  |  | 14.1 |  |  | 14.1 |
Total assets at fair value | $ | 162.8 |  |  | $ | 301.0 |  |  | $ | 14.1 |  |  | 477.9 |
Assets measured at net asset value practical expedient (1) |  |  |  |  |  |  |
Private mutual funds |  |  |  |  |  |  | 186.1 |
Common collective trusts |  |  |  |  |  |  | 109.0 |
|  |  |  |  |  |  | 295.1 |
Total plan assets |  |  |  |  |  |  | $ | 773.0 |
(1) In accordance with Accounting Standards Update 2015-07, investments that are measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the total value of assets of the plans.
Summary of changes in the fair value of the Master Trust's Level 3 investments | The table below sets forth a summary of changes in the fair value of the Master Trust’s Level 3 investments:
| Successor | Predecessor
| April 2 through December 31, 2017 | January 1 through April 1, 2017 |  | Year Ended December 31, 2016
| (Dollars in millions)
Balance, beginning of period | $ | 13.8 |  | $ | 14.1 |  |  | $ | 23.0 |
Realized gains | — |  | 0.6 |  |  | 1.8 |
Unrealized gains (losses) relating to investments still held at the reporting date | 2.2 |  | (0.6 | ) |  | 0.2 |
Purchases, sales and settlements, net | (4.2 | ) | (0.3 | ) |  | (10.9 | )
Balance, end of period | $ | 11.8 |  | $ | 13.8 |  |  | $ | 14.1 |
Summary of estimated future benefit payments | The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid in connection with the Company’s benefit obligation:
| Successor
| Pension Benefits
| (Dollars in millions)
2018 | $ | 57.2 |
2019 | 57.2 |
2020 | 58.6 |
2021 | 59.5 |
2022 | 58.9 |
Years 2023-2027 | 280.1 |

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