Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension plan
Cost components:
Service cost	$ (223)	$ (265)	$ (248)
Interest cost	(406)	(466)	(392)
Expected return on plan assets	517	615	576
Net amortization and deferral	(184)	(284)	(45)
Settlement recognition	0	(536)	(407)
Total net periodic pension cost	(296)	(936)	(516)
SERP
Cost components:
Service cost	0	0	(26)
Interest cost	(323)	(278)	(258)
Net amortization and deferral	(343)	(337)	(23)
Total net periodic pension cost	$ (666)	$ (615)	$ (307)