Employee Retirement Plans - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee retirement plans
Total pension plan assets	$ 38,053	$ 33,903
Level 2
Employee retirement plans
Total pension plan assets	37,397	31,877
Level 3
Employee retirement plans
Total pension plan assets	656	2,026	$ 1,748
Equity holdings
Employee retirement plans
Total pension plan assets		13,384
Equity holdings | Level 2
Employee retirement plans
Total pension plan assets		13,384
Fixed-income holdings
Employee retirement plans
Total pension plan assets	37,397	18,493
Fixed-income holdings | Level 2
Employee retirement plans
Total pension plan assets	37,397	18,493
Alternative investments
Employee retirement plans
Total pension plan assets	656	2,026
Alternative investments | Level 3
Employee retirement plans
Total pension plan assets	656	2,026
Pension plan
Employee retirement plans
Total pension plan assets	$ 38,053	$ 33,903	$ 29,223