Commitments and Contingencies - Collective Bargaining Agreements and Environmental Contingencies (Details)	12 Months Ended
	Dec. 31, 2018 employee item location
Employees covered by collective bargaining agreements
Collective Bargaining Agreements
Number of Location Entity Operates | location	51
Expiration period of collective bargaining agreements	7 years
Total employees | Employees covered by collective bargaining agreements
Collective Bargaining Agreements
Percentage of employees covered by collective bargaining agreements	11.00%
Number of employees | employee	1,780
Number of collective bargaining agreements that expire over the next seven years | item	40
Employees covered by collective bargaining agreements that expire during 2019
Collective Bargaining Agreements
Number of employees | employee	400
Employees covered by collective bargaining agreements that expire during 2019 | Employees covered by collective bargaining agreements
Collective Bargaining Agreements
Number of collective bargaining agreements that expire within one year | item	13