CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Surplus	Accumulated Other Comprehensive Income (Loss)	Total
BALANCE at Dec. 31, 2015	$ 113	$ 150,305	$ 28,545	$ (5,101)	$ 173,862
Components of comprehensive income:
Net income			19,922		19,922
Foreign currency translation adjustments				(1,566)	(1,566)
Total comprehensive income			19,922	(1,566)	18,356
Issuance of common stock to non-employee directors (4,410, 5,922 and 5,814 for the period ended December 31, 2016, 2017 and 2018, respectively)		96			96
Exercise of stock options (500, 26,500 and 10,250 for the period ended December 31, 2016, 2017 and 2018, respectively)		3			3
Dividends paid ($0.68, $0.72 and $0.72 per share for the period ended December 31, 2016, 2017 and 2018, respectively)			(7,715)		(7,715)
BALANCE at Dec. 31, 2016	113	150,404	40,752	(6,667)	184,602
Components of comprehensive income:
Net income			23,016		23,016
Foreign currency translation adjustments				3,374	3,374
Total comprehensive income			23,016	3,374	26,390
Issuance of common stock to non-employee directors (4,410, 5,922 and 5,814 for the period ended December 31, 2016, 2017 and 2018, respectively)		150			150
Exercise of stock options (500, 26,500 and 10,250 for the period ended December 31, 2016, 2017 and 2018, respectively)	1	145			146
Dividends paid ($0.68, $0.72 and $0.72 per share for the period ended December 31, 2016, 2017 and 2018, respectively)			(8,188)		(8,188)
BALANCE at Dec. 31, 2017	114	150,699	55,580	(3,293)	203,100
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment for adoption of ASU 2014-09 (January 1, 2018)			(324)		(324)
BALANCE (January 1, 2018)	114	150,699	55,256	(3,293)	202,776
Prior period accounting reclassification			552	(552)
Components of comprehensive income:
Net income			33,746		33,746
Foreign currency translation adjustments				(965)	(965)
Total comprehensive income			34,298	(1,517)	32,781
Issuance of common stock to non-employee directors (4,410, 5,922 and 5,814 for the period ended December 31, 2016, 2017 and 2018, respectively)		150			150
Exercise of stock options (500, 26,500 and 10,250 for the period ended December 31, 2016, 2017 and 2018, respectively)		56			56
Dividends paid ($0.68, $0.72 and $0.72 per share for the period ended December 31, 2016, 2017 and 2018, respectively)			(8,200)		(8,200)
BALANCE at Dec. 31, 2018	$ 114	$ 150,905	$ 81,354	$ (4,810)	$ 227,563