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Commitments and Contingencies - Collective Bargaining Agreements and Environmental Contingencies (Details)
 12 Months Ended
Dec. 31, 2017
employee
item
location
Employees covered by collective bargaining agreements  
Collective Bargaining Agreements  
Number of Location Entity Operates | location 53
Expiration period of collective bargaining agreements 8 years
Total employees | Employees covered by collective bargaining agreements  
Collective Bargaining Agreements  
Percentage of employees covered by collective bargaining agreements 12.00%
Number of employees | employee 1,770
Number of collective bargaining agreements that expire over the next five years | item 41
Employees covered by collective bargaining agreements that expire during 2018  
Collective Bargaining Agreements  
Number of employees | employee 600
Employees covered by collective bargaining agreements that expire during 2018 | Employees covered by collective bargaining agreements  
Collective Bargaining Agreements  
Number of collective bargaining agreements that expire within one year | item 23