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Pension and Savings Plans Changes in the Fair Value of Master Trust's Level 3 Investments (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2016
Predecessor | Real Estate Investment [Member] |  |  |
Summary of changes in the fair value of the Master Trust's investments |  |  |
Fair value of plan assets at beginning of period | $ 14.1 |  |
Assets Held At Reporting Date: |  |  |
Fair value of plan assets at end of period |  |  | $ 14.1
Predecessor | Level 3 [Member] |  |  |
Summary of changes in the fair value of the Master Trust's investments |  |  |
Fair value of plan assets at beginning of period | 14.1 | $ 13.8 | 23.0
Assets Held At Reporting Date: |  |  |
Realized gains | 0.6 |  | 1.8
Unrealized gains relatng to investments still held at the reporting date | (0.6) |  | 0.2
Purchases, sales and settlements, net | (0.3) |  | (10.9)
Fair value of plan assets at end of period | 13.8 |  | 14.1
Predecessor | Level 3 [Member] | Real Estate Investment [Member] |  |  |
Summary of changes in the fair value of the Master Trust's investments |  |  |
Fair value of plan assets at beginning of period | 14.1 |  |
Assets Held At Reporting Date: |  |  |
Fair value of plan assets at end of period |  |  | 14.1
Predecessor | Pension Plans, Defined Benefit [Member] |  |  |
Summary of changes in the fair value of the Master Trust's investments |  |  |
Fair value of plan assets at beginning of period | 773.0 |  | 757.3
Assets Held At Reporting Date: |  |  |
Fair value of plan assets at end of period |  |  | $ 773.0
Successor |  |  |
Summary of changes in the fair value of the Master Trust's investments |  |  |
Fair value of plan assets at beginning of period |  | 0.0 |
Assets Held At Reporting Date: |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Successor | Real Estate Investment [Member] |  |  |
Assets Held At Reporting Date: |  |  |
Fair value of plan assets at end of period |  | 11.8 |
Successor | Level 3 [Member] |  |  |
Summary of changes in the fair value of the Master Trust's investments |  |  |
Fair value of plan assets at beginning of period |  | 13.8 |
Assets Held At Reporting Date: |  |  |
Realized gains |  | 0.0 |
Unrealized gains relatng to investments still held at the reporting date |  | 2.2 |
Purchases, sales and settlements, net |  | (4.2) |
Fair value of plan assets at end of period | 13.8 | 11.8 |
Successor | Level 3 [Member] | Real Estate Investment [Member] |  |  |
Assets Held At Reporting Date: |  |  |
Fair value of plan assets at end of period |  | 11.8 |
Successor | Pension Plans, Defined Benefit [Member] |  |  |
Summary of changes in the fair value of the Master Trust's investments |  |  |
Fair value of plan assets at beginning of period |  | 783.1 |
Assets Held At Reporting Date: |  |  |
Fair value of plan assets at end of period | $ 783.1 | $ 776.6 |

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