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Postretirement Health Care and Life Insurance Benefits Pre-Tax Amounts Recorded in Accumulated Other Comprehensive Loss (Details) - Postretirement Health Care and Life Insurance Benefits $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Apr. 01, 2017 USD ($) | Dec. 31, 2018 USD ($) years | Dec. 31, 2016 USD ($) years | Dec. 31, 2019 USD ($)
Successor |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Net actuarial loss arising during year |  | $ 0.0 |  |
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax |  | (51.7) |  |
Amortization: |  |  |  |
Actuarial loss |  | 0.0 |  |
Prior service credit |  | 0.0 |  |
Settlement related to the Patriot bankruptcy: |  |  |  |
Prior service credit |  | 0.0 |  |
Total recorded in “Accumulated other comprehensive income” |  | $ (51.7) |  |
Remaining service period for remaining eligible employees | years |  | 5.9 |  |
Predecessor |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Net actuarial loss arising during year | $ 0.0 |  | $ 32.3 |
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax | 0.0 |  | 0.0 |
Amortization: |  |  |  |
Actuarial loss | (5.5) |  | (20.4) |
Prior service credit | 2.3 |  | 9.2 |
Settlement related to the Patriot bankruptcy: |  |  |  |
Prior service credit | 0.0 |  | 7.2 |
Total recorded in “Accumulated other comprehensive income” | $ (3.2) |  | $ 28.3 |
Percentage of actuarial gains and losses amortized | 0.00% |  |  |
Remaining service period for remaining eligible employees | years |  |  | 10.3 |
Scenario, Forecast |  |  |  |
Settlement related to the Patriot bankruptcy: |  |  |  |
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year |  |  |  | $ 8.8

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