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Pension and Savings Plans Fair Value of Assets in the Master Trust by Asset (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Apr. 01, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Predecessor |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value |  |  | $ 295.1 |
Predecessor | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 162.8 |
Predecessor | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 301.0 |
Predecessor | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  | $ 13.8 | 14.1 | $ 23.0
Predecessor | Mutual funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 119.9 |
Predecessor | Mutual funds | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 119.9 |
Predecessor | Mutual funds | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Mutual funds | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | International equity securities | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | U.S. government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 47.8 |
Predecessor | U.S. government securities | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 25.1 |
Predecessor | U.S. government securities | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 22.7 |
Predecessor | International government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 12.6 |
Predecessor | International government securities | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | International government securities | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 12.6 |
Predecessor | International government securities | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Common collective trusts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 109.0 |
Predecessor | Corporate bonds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 265.7 |
Predecessor | Corporate bonds | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Corporate bonds | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 265.7 |
Predecessor | Corporate bonds | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Cash funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 17.8 |
Predecessor | Cash funds | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 17.8 |
Predecessor | Cash funds | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Cash funds | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Real estate investment trusts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 14.1 |
Predecessor | Real estate investment trusts | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Real estate investment trusts | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 0.0 |
Predecessor | Real estate investment trusts | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 14.1 |
Predecessor | Private mutual funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 186.1 |
Predecessor | Pension Plans, Defined Benefit [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | 773.0 | $ 757.3
Predecessor | Fair Value Pension Plan Assets [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period |  |  | $ 477.9 |
Successor |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | $ 0.0 | 0.0 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value | 287.9 |  |  |
Successor | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 146.3 |  |  |
Successor | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 330.6 |  |  |
Successor | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 11.8 | 13.8 |  |
Successor | Mutual funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 108.0 |  |  |
Successor | Mutual funds | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 108.0 |  |  |
Successor | Mutual funds | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Mutual funds | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | U.S. government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 29.3 |  |  |
Successor | U.S. government securities | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 7.5 |  |  |
Successor | U.S. government securities | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 21.8 |  |  |
Successor | U.S. government securities | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | International government securities |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 17.7 |  |  |
Successor | International government securities | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | International government securities | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 17.7 |  |  |
Successor | International government securities | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Common collective trusts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 107.5 |  |  |
Successor | Corporate bonds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 291.1 |  |  |
Successor | Corporate bonds | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Corporate bonds | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 291.1 |  |  |
Successor | Corporate bonds | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Cash funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 30.8 |  |  |
Successor | Cash funds | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 30.8 |  |  |
Successor | Cash funds | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Cash funds | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Real estate investment trusts |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 11.8 |  |  |
Successor | Real estate investment trusts | Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Real estate investment trusts | Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 0.0 |  |  |
Successor | Real estate investment trusts | Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 11.8 |  |  |
Successor | Private mutual funds |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 180.4 |  |  |
Successor | Pension Plans, Defined Benefit [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | 776.6 | $ 783.1 |  |
Successor | Fair Value Pension Plan Assets [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Fair value of plan assets at end of period | $ 488.7 |  |  |

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