Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Balance at Dec. 31, 2014	$ 336,973	$ 481	$ 201,389	$ (46)	$ 261,150	$ (126,001)
Balance, shares at Dec. 31, 2014		48,148				(14,679)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	64,869				64,869
Issuance of shares pursuant to stock compensation plan	$ 23,160		4,332			$ 18,828
Issuance of shares pursuant to stock compensation plan, shares	1,118					1,118
Stock compensation	$ 20,182		20,182
Issuance of shares pursuant to employee stock purchase plan	$ 4,671		3,879			$ 792
Issuance of shares pursuant to employee stock purchase plan, shares	43					43
Federal income tax benefit related to exercise of stock options	$ 45,314		45,314
Treasury stock purchases	$ (645)					$ (645)
Treasury stock purchases, shares	(5)					(5)
Issuance of shares for acquisitions	$ 364,333		332,659			$ 31,674
Issuance of shares for acquisitions, shares	2,149					2,149
Balance at Dec. 31, 2015	$ 858,857	$ 481	607,755	(46)	326,019	$ (75,352)
Balance, shares at Dec. 31, 2015		48,148				(11,374)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	109,857				109,857
Issuance of shares pursuant to stock compensation plan	$ 23,527		(82,273)			$ 105,800
Issuance of shares pursuant to stock compensation plan, shares	827					827
Stock compensation	$ 29,747		29,747
Issuance of shares pursuant to employee stock purchase plan	$ 6,236		1,434			$ 4,802
Issuance of shares pursuant to employee stock purchase plan, shares	47					47
Treasury stock purchases	$ (112,699)					$ (112,699)
Treasury stock purchases, shares	(882)					(882)
Balance at Dec. 31, 2016	$ 915,525	$ 481	556,663	(46)	435,876	$ (77,449)
Balance, shares at Dec. 31, 2016		48,148				(11,382)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	163,945				163,945
Issuance of shares pursuant to stock compensation plan	$ 49,845		28,174			$ 21,671
Issuance of shares pursuant to stock compensation plan, shares	1,113					1,113
Stock compensation	$ 37,348		37,348
Issuance of shares pursuant to employee stock purchase plan	$ 7,044		4,682			$ 2,362
Issuance of shares pursuant to employee stock purchase plan, shares	51					51
Treasury stock purchases	$ (6,613)					$ (6,613)
Treasury stock purchases, shares	(44)					(44)
Balance at Dec. 31, 2017	$ 1,167,094	$ 481	$ 626,867	$ (46)	$ 599,821	$ (60,029)
Balance, shares at Dec. 31, 2017		48,148				(10,262)