Employee Retirement Plans - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Employee retirement plans
Total pension plan assets	$ 38,053
Level 2
Employee retirement plans
Total pension plan assets	37,397
Level 3
Employee retirement plans
Total pension plan assets	656	$ 2,026
Fixed-income holdings
Employee retirement plans
Total pension plan assets	37,397
Fixed-income holdings | Level 2
Employee retirement plans
Total pension plan assets	37,397
Alternative investments
Employee retirement plans
Total pension plan assets	656
Alternative investments | Level 3
Employee retirement plans
Total pension plan assets	656
Pension plan
Employee retirement plans
Total pension plan assets	$ 38,053	$ 33,903