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Postretirement Health Care and Life Insurance Benefits Pre-Tax Amounts Recorded in Accumulated Other Comprehensive Loss (Details) - Predecessor - Postretirement Health Care and Life Insurance Benefits [Member] $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Apr. 01, 2017 USD ($) | Dec. 31, 2016 USD ($) years | Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net actuarial loss arising during period | $ 0.0 | $ 32.3 | $ (35.1)
Amortization: |  |  |
Actuarial loss | (5.5) | (20.4) | (24.9)
Prior service credit (cost) | 2.3 | 9.2 | 6.8
Settlement related to the Patriot bankruptcy |  |  |
Prior service cost | 0.0 | 7.2 | (16.6)
Total recorded in other comprehensive (income) loss | $ (3.2) | $ 28.3 | $ (69.8)
Percentage of actuarial gains and losses amortized | 0.00% |  |
Defined Benefit Plan, Amortization Period | years |  | 10.31 |

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