Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Total
Balance at Dec. 31, 2014	$ 295,202	$ (2,651)	$ 2,058	$ 0	$ 97,289	$ 391,898
Balance (in shares) at Dec. 31, 2014			30,845,586	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	59,722	0	$ 0	$ 0	0	59,722
Other comprehensive income (loss)	0	(2,463)	0	0	0	(2,463)
Dividends	(10,727)	0	0	0	0	(10,727)
Treasury shares purchased	0	0	$ 0	$ (1,205)	0	(1,205)
Treasury shares purchased (in shares)			0	(20,692)
Shares and options issued under stock plans and an income tax benefit	0	0	$ 44	$ 1,131	25,305	26,480
Shares and options issued under stock plans and an income tax benefit (in shares)			682,863	19,603
Balance at Dec. 31, 2015	344,197	(5,114)	$ 2,102	$ (74)	122,594	463,705
Balance (in shares) at Dec. 31, 2015			31,528,449	(1,089)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	55,972	0	$ 0	$ 0	0	55,972
Other comprehensive income (loss)	0	(1,735)	0	0	0	(1,735)
Dividends	(12,080)	0	0	0	0	(12,080)
Treasury shares purchased	0	0	$ 0	$ (1,588)	0	(1,588)
Treasury shares purchased (in shares)			0	(24,912)
Shares and options issued under stock plans and an income tax benefit	0	0	$ 15	$ 1,662	15,082	16,759
Shares and options issued under stock plans and an income tax benefit (in shares)			229,412	26,001
Balance at Dec. 31, 2016	388,089	(6,849)	$ 2,117	$ 0	137,676	521,033
Balance (in shares) at Dec. 31, 2016			31,757,861	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	90,071	0	$ 0	$ 0	0	90,071
Other comprehensive income (loss)	(57)	5,207	0	0	0	5,150
Dividends	(13,464)	0	0	0	0	(13,464)
Treasury shares purchased	0	0	$ 0	$ (1,905)	0	(1,905)
Treasury shares purchased (in shares)			0	(23,182)
Shares and options issued under stock plans and an income tax benefit	0	0	$ 18	$ 1,905	14,073	15,996
Shares and options issued under stock plans and an income tax benefit (in shares)			261,744	23,182
Balance at Dec. 31, 2017	$ 464,639	$ (1,642)	$ 2,135	$ 0	$ 151,749	$ 616,881
Balance (in shares) at Dec. 31, 2017			32,019,605	0