Consolidated Statements of Stockholders Equity - USD ($) $ in Thousands	Total	Common Stock Shares [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, Shares at Dec. 31, 2014		15,595,000
Balance at Dec. 31, 2014	$ 508,252		$ 1,559	$ 107,336	$ 0	$ 408,911	$ (9,554)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock and exercise of stock options, shares issued		475,000
Treasury Stock, Shares		(18,000)
Employee stock purchase plan, shares issued		34,000
Issuance of restricted stock and exercise of stock options, including excess income tax benefits	24,372		46	22,715	1,611
Purchase of treasury stock,treasury stock acquired in lieu of cash payment on stock option exercises and income tax withholding obligations	(1,523)			88	(1,611)
Issuance of common stock for acquisitions	0
Proceeds from employee stock purchase plan	2,162		3	2,159	0
Stock compensation	12,827			12,827
Foreign Currency Translation Adjustment	(11,204)						(11,204)
Net Income	44,985					44,985
Balance at Dec. 31, 2015	579,871		1,608	145,125	0	453,896	(20,758)
Balance, Shares at Dec. 31, 2015		16,086,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock and exercise of stock options, shares issued		416,000
Treasury Stock, Shares		(195,000)
Employee stock purchase plan, shares issued		31,000
Issuance of restricted stock and exercise of stock options, including excess income tax benefits	17,346		22	103	17,221
Purchase of treasury stock,treasury stock acquired in lieu of cash payment on stock option exercises and income tax withholding obligations	(17,235)			0	(17,235)
Issuance of common stock for acquisitions	0
Proceeds from employee stock purchase plan	2,361		3	2,358	0
Stock compensation	15,242			15,242
Foreign Currency Translation Adjustment	(514)						(514)
Net Income	63,084					63,084
Balance at Dec. 31, 2016	$ 660,155		1,633	162,828	(14)	516,980	(21,272)
Balance, Shares at Dec. 31, 2016	16,337,670	16,338,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock and exercise of stock options, shares issued		676,000
Treasury Stock, Shares		(27,000)
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares		3,200,000
Employee stock purchase plan, shares issued		23,000
Issuance of restricted stock and exercise of stock options, including excess income tax benefits	$ 32,003		66	27,866	4,071
Purchase of treasury stock,treasury stock acquired in lieu of cash payment on stock option exercises and income tax withholding obligations	(4,057)			0	(4,057)
Issuance of common stock for acquisitions	413,139		320	412,819
Proceeds from employee stock purchase plan	2,705		2	2,703	0
Stock compensation	19,352			19,352
Foreign Currency Translation Adjustment	6,313						6,313
Net Income	68,644					68,644
Balance at Dec. 31, 2017	$ 1,198,254		$ 2,021	$ 625,568	$ 0	$ 585,624	$ (14,959)
Balance, Shares at Dec. 31, 2017	20,210,054	20,210,000