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Pension and Savings Plans Fair Value of Assets in the Master Trust by Asset (Details) - Successor - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Apr. 01, 2017
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value |  | $ 287.9 |
Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period |  | 108.0 |
U.S. government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | $ 198.9 | 29.3 |
International government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 12.4 | 17.7 |
Commercial Paper |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 2.1 |  |
Common collective trusts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period |  | 107.5 |
Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 466.1 | 291.1 |
Cash funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 38.4 | 30.8 |
Real estate interests |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 6.2 | 11.8 |
Private mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 40.7 | 180.4 |
Level 1 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 219.9 | 146.3 |
Level 1 | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period |  | 108.0 |
Level 1 | U.S. government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 181.5 | 7.5 |
Level 1 | International government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 1 | Commercial Paper |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 |  |
Level 1 | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 1 | Cash funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 38.4 | 30.8 |
Level 1 | Real estate interests |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 2 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 498.0 | 330.6 |
Level 2 | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period |  | 0.0 |
Level 2 | U.S. government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 17.4 | 21.8 |
Level 2 | International government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 12.4 | 17.7 |
Level 2 | Commercial Paper |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 2.1 |  |
Level 2 | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 466.1 | 291.1 |
Level 2 | Cash funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 2 | Real estate interests |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 3 |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 6.2 | 11.8 | $ 13.8
Level 3 | Mutual funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period |  | 0.0 |
Level 3 | International equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period |  | 0.0 |
Level 3 | U.S. government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 |  |
Level 3 | International government securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 3 | Commercial Paper |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 |  |
Level 3 | Corporate bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 3 | Cash funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 0.0 | 0.0 |
Level 3 | Real estate interests |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 6.2 | 11.8 |
Pension Plans, Defined Benefit |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | 764.8 | 776.6 | $ 783.1
Fair Value Pension Plan Assets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets at end of period | $ 724.1 | $ 488.7 |

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