Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension plan
Cost components:
Service cost-benefits earned during the period	$ (61)	$ (56)
Interest cost on projected benefit obligation	(104)	(102)
Expected return on plan assets	129	129
Net amortization and deferral	(50)	(46)
Total net periodic pension cost	(86)	(75)
SERP
Cost components:
Interest cost on projected benefit obligation	(76)	(81)
Net amortization and deferral	(93)	(86)
Total net periodic pension cost	$ (169)	$ (167)