Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Pension plan
Cost components:
Service cost-benefits earned during the period	$ (30)	$ (167)
Interest cost on projected benefit obligation	(292)	(305)
Expected return on plan assets	397	387
Net amortization and deferral	(74)	(138)
Total net periodic pension cost	1	(223)
SERP
Cost components:
Interest cost on projected benefit obligation	(216)	(242)
Net amortization and deferral	(189)	(257)
Total net periodic pension cost	$ (405)	$ (499)