Consolidated Statement of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2014	$ 141,602	$ 209	$ 124,942	$ 117,318	$ (99,254)	$ (1,613)
Beginning Balance, Shares at Dec. 31, 2014		20,856			2,703
Issuance of common stock under stock plans, including tax effects	1,680	$ 2	1,678
Issuance of common stock under stock plans, including tax effects, Shares		207
Stock-based compensation expense	8,347		8,347
Repurchases of common stock	(20,931)				$ (20,931)
Repurchase of common stock, Shares					608
Dividends paid on common shares	(12,179)			(12,179)
Net income	11,996			11,996
Net change in marketable investments, net of tax	(26)					(26)
Foreign currency translation	(3,187)					(3,187)
Ending Balance at Dec. 31, 2015	127,302	$ 211	134,967	117,135	$ (120,185)	(4,826)
Ending Balance, Shares at Dec. 31, 2015		21,063			3,311
Issuance of common stock under stock plans, including tax effects	14,632	$ 6	14,626
Issuance of common stock under stock plans, including tax effects, Shares		656
Stock-based compensation expense	7,976		7,976
Repurchases of common stock	(1,791)				$ (1,791)
Repurchase of common stock, Shares					47
Dividends paid on common shares	(12,987)			(12,987)
Net income	17,651			17,651
Net change in marketable investments, net of tax	17					17
Foreign currency translation	(2,764)					(2,764)
Ending Balance at Dec. 31, 2016	150,036	$ 217	157,569	121,799	$ (121,976)	(7,573)
Ending Balance, Shares at Dec. 31, 2016		21,719			3,358
Issuance of common stock under stock plans, including tax effects	15,979	$ 7	15,972
Issuance of common stock under stock plans, including tax effects, Shares		713
Cumulative effect adjustment due to adoption of new accounting pronouncements	(419)		(121)	(298)
Stock-based compensation expense	8,490		8,490
Repurchases of common stock	(39,967)				$ (39,967)
Repurchase of common stock, Shares					1,033
Dividends paid on common shares	(13,631)			(13,631)
Net income	15,140			15,140
Net change in marketable investments, net of tax	(32)					(32)
Foreign currency translation	5,593					5,593
Ending Balance at Dec. 31, 2017	$ 141,189	$ 224	$ 181,910	$ 123,010	$ (161,943)	$ (2,012)
Ending Balance, Shares at Dec. 31, 2017		22,432			4,391