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Postretirement Health Care and Life Insurance Benefits Net Periodic Postretirement Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Apr. 01, 2017
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2016
Successor        
Components of net periodic postretirement benefit cost        
Net periodic postretirement benefit cost   $ 21.9 $ 18.1  
Successor | Postretirement Health Care and Life Insurance Benefits        
Components of net periodic postretirement benefit cost        
Service cost for benefits earned   6.9 8.2  
Interest cost on accumulated postretirement benefit obligation   24.2 28.3  
Amortization of prior service credit   0.0 0.0  
Amortization of actuarial loss   0.0 0.0  
Net actuarial gain   (22.0) (128.4)  
Net periodic postretirement benefit cost   $ 9.1 $ (91.9)  
Predecessor        
Components of net periodic postretirement benefit cost        
Net periodic postretirement benefit cost $ 14.4     $ 57.1
Predecessor | Postretirement Health Care and Life Insurance Benefits        
Components of net periodic postretirement benefit cost        
Service cost for benefits earned 2.3     10.4
Interest cost on accumulated postretirement benefit obligation 8.4     34.5
Amortization of prior service credit (2.3)     (9.2)
Amortization of actuarial loss 5.5     20.4
Net actuarial gain 0.0     0.0
Net periodic postretirement benefit cost $ 13.9     $ 56.1