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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 ($) | Dec. 31, 2020 USD ($) years | Dec. 31, 2019 USD ($) years | Dec. 31, 2018 USD ($) | Dec. 31, 2017 years
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax | $ 0.0 |  | $ 0.0 | $ (51.7) |
Amortization: |  |  |  |  |
Actuarial loss | (5.5) |  | 0.0 | 0.0 |
Prior service credit | 2.3 |  | 8.7 | 0.0 |
Settlement related to the Patriot bankruptcy: |  |  |  |  |
Total recorded in “Accumulated other comprehensive income” | $ (3.2) |  | $ 8.7 | $ (51.7) |
Percentage of actuarial gains and losses amortized | 0.00% |  |  |  |
Remaining service period for remaining eligible employees | years |  |  | 5.9 |  | 10.3
Scenario, Forecast |  |  |  |  |
Settlement related to the Patriot bankruptcy: |  |  |  |  |
Remaining service period for remaining eligible employees | years |  | 4.9 |  |  |
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year |  | $ 8.7 |  |  |

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