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Postretirement Health Care and Life Insurance Benefits Assupmtions Used to Determine the Benefit Obligations and Periodic Benefit Cost (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Apr. 01, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Postretirement Health Care and Life Insurance Benefits [Member]        
Assumed health care cost trend rates, One percentage point increase        
One Percentage-Point Increase Effect on total service and interest cost components $ 3.2      
One Percentage-Point Decrease Effect on total service and interest cost component (2.9)      
One Percentage-Point Increase Effect on total postretirement benefit obligation 69.1      
One Percentage-Point Decrease Effect on total postretirement benefit obligation $ (60.7)      
Pre-Medicare [Member]        
Assumed health care cost trend rate        
Health care cost trend rate assumed for next year   7.00% 6.20%  
Rate to which the cost trend is assumed to decline (the ultimate trend rate)   4.75% 4.75%  
Year that the rate reaches the ultimate trend rate 2023   2021  
Post-Medicare [Member]        
Assumed health care cost trend rate        
Health care cost trend rate assumed for next year   6.50% 5.60%  
Rate to which the cost trend is assumed to decline (the ultimate trend rate)   4.75% 4.75%  
Year that the rate reaches the ultimate trend rate 2023   2021  
Successor | Postretirement Health Care and Life Insurance Benefits [Member]        
Weighted-average assumptions used to determine benefit obligations        
Discount rate   3.70%    
Measurement date Dec. 31, 2017      
Weighted-average assumptions used to determine net periodic benefit cost        
Discount rate   4.10%    
Predecessor | Postretirement Health Care and Life Insurance Benefits [Member]        
Weighted-average assumptions used to determine benefit obligations        
Discount rate     4.15%  
Measurement date     Dec. 31, 2016  
Weighted-average assumptions used to determine net periodic benefit cost        
Discount rate 4.15%   4.50% 4.10%