Employee Retirement Plans - Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee retirement plans
Total pension plan assets	$ 29,223	$ 26,378
Level 2
Employee retirement plans
Total pension plan assets	27,475	25,014
Level 3
Employee retirement plans
Total pension plan assets	1,748	1,364	$ 1,316
Equity holdings
Employee retirement plans
Total pension plan assets	15,654	14,460
Equity holdings | Level 2
Employee retirement plans
Total pension plan assets	15,654	14,460
Fixed-income holdings
Employee retirement plans
Total pension plan assets	11,821	10,554
Fixed-income holdings | Level 2
Employee retirement plans
Total pension plan assets	11,821	10,554
Alternative investments
Employee retirement plans
Total pension plan assets	1,748	1,364
Alternative investments | Level 3
Employee retirement plans
Total pension plan assets	1,748	1,364
Pension plan
Employee retirement plans
Total pension plan assets	$ 29,223	$ 26,378	$ 26,046