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Postretirement Benefit Plans (Details)
 5 Months Ended 7 Months Ended 12 Months Ended 19 Months Ended
May 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
employee
Dec. 31, 2017
USD ($)
employee
Dec. 31, 2016
USD ($)
employee
Dec. 31, 2015
USD ($)
employee
May 31, 2015
USD ($)
Dec. 31, 2016
USD ($)
employee
Multiemployer Defined Contributory Trust Plan [Member]              
Multiemployer Plans [Line Items]              
Number of employees in plan | employee   1,365 1,365 1,369 1,431   1,369
Required employer's contribution per hour per active employee     $ 0.80 $ 0.80 $ 0.75    
Required defined contributory trust contribution, percent of earnings     4.50% 4.00% 4.00%    
Period Contributions     $ 2,800,000 $ 2,800,000 $ 2,800,000    
Multiemployer Health and Welfare Plan [Member]              
Multiemployer Plans [Line Items]              
Number of employees in plan | employee   649 649 658 651   658
Required employer's contribution per hour per active employee $ 5.80 $ 6.10       $ 5.25 $ 5.50
Period Contributions     $ 8,100,000 $ 7,600,000 $ 7,600,000