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, 2015		16,086,000
Balance at Dec. 31, 2015	$ 579,871		$ 1,608	$ 145,125	$ 0	$ 453,896	$ (20,758)
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		16,338,000
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of restricted stock and exercise of stock options, shares issued		307,000
Treasury Stock, Shares		(26,000)
Issuance of restricted stock and exercise of stock options, including excess income tax benefits	$ 14,275		28	8,090	6,157
Purchase of treasury stock,treasury stock acquired in lieu of cash payment on stock option exercises and income tax withholding obligations	(6,252)			0	(6,252)
Proceeds from employee stock purchase plan	0
Stock compensation	24,241			24,241
Foreign Currency Translation Adjustment	(1,986)						(1,986)
Net Income	28,793					28,793
Balance at Dec. 31, 2018	$ 1,263,655		$ 2,049	$ 657,899	$ (95)	620,747	$ (16,945)
Balance, Shares at Dec. 31, 2018	20,491,229	20,491,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 6,330					$ 6,330