Labor Union Contracts (Details)	6 Months Ended
Jun. 30, 2018
Concentration risk
Number of unionized employees (less than)	300
Total number of employees \| Unionized employees concentration risk
Concentration risk
Concentration risk, percent	28.00%
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year \| Unionized employees concentration risk
Concentration risk
Concentration risk, percent	1.00%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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