Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension plan
Cost components:
Service cost-benefits earned during the period	$ 0	$ (61)
Interest cost on projected benefit obligation	(91)	(104)
Expected return on plan assets	146	129
Net amortization and deferral	(16)	(50)
Total net periodic pension cost	39	(86)
SERP
Cost components:
Interest cost on projected benefit obligation	(64)	(76)
Net amortization and deferral	(16)	(93)
Total net periodic pension cost	$ (80)	$ (169)