Share-Based Plans - Restricted Stock Activity (Details) - Restricted Stock - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Unvested, beginning of period (in shares)	137,880	141,755	137,200
Granted (in shares)	30,185	29,625	32,630
Vested (in shares)	(26,450)	(31,810)	(26,000)
Cancelled (in shares)	(2,160)	(1,690)	(2,075)
Unvested, end of period (in shares)	139,455	137,880	141,755
Weighted average grant date fair value
Unvested, beginning of period (in dollars per share)	$ 43.52	$ 39.77	$ 36.50
Granted (in dollars per share)	52.82	52.73	43.48
Vested (in dollars per share)	42.41	35.42	27.30
Cancelled (in dollars per share)	42.54	43.16	38.59
Unvested, end of period (in dollars per share)	$ 45.75	$ 43.52	$ 39.77
Additional information
Compensation expense	$ 1,350	$ 1,450	$ 1,220
Employees
Summary of restricted stock awards
Vesting period	5 years
Nonemployee Directors
Summary of restricted stock awards
Vesting period	3 years
Amended 2004 Plan and 2013 Plan
Additional information
Unrecognized compensation expense	$ 3,170