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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