Retirement Plans (Changes in Benefit Obligations, Components of Benefit Costs, Weighted-Average Assumptions, and Plan Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assumptions (end of period):
Discount rate used in determining benefit obligation	3.45%	3.88%	4.12%
Reconciliation of funded status (end of period):
Accumulated benefit obligation	$ (12,758)	$ (12,618)	$ (12,915)
Projected benefit obligation	(12,758)	(12,618)	(12,915)
Fair value of plan assets	11,153	10,415	10,419
Defined benefit plan, funded status of plan, total	(1,605)	(2,203)	(2,496)
Net actuarial loss	(3,554)	(4,102)	(3,957)
Accumulated other comprehensive loss	(3,554)	(4,102)	(3,957)
Prepaid pension cost	1,949	1,899	1,461
Net amount recognized as of December 31	(1,605)	(2,203)	(2,496)
Changes in projected benefit obligation:
Benefit obligation as of beginning of period	12,618	12,915	13,909
Interest cost	471	512	518
Actuarial liability loss	554	19	(646)
Benefits paid	(885)	(828)	(866)
Benefit obligation as of end of period	12,758	12,618	12,915	$ 13,909
Change in plan assets:
Fair value of plan assets as of beginning of period	10,415	10,419	11,290
Actual return on plan assets (net of expenses)	1,623	415	(5)
Employer contributions		409
Benefits paid	885	828	866
Fair value of plan assets as of end of period	11,153	10,415	10,419	11,290
Net periodic pension cost:
Service cost - benefits earned during the period
Interest cost on projected benefit obligation	471	512	518
Expected return on plan assets	(646)	(653)	(701)
Recognized actuarial loss	126	112	103
Net periodic pension cost	$ (49)	$ (29)	$ (80)
Discount rate	3.88%	4.12%	3.86%
Expected return on plan assets	6.50%	6.50%	6.50%