Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2012	$ 145,299	$ 481	$ 154,018	$ (268)	$ 163,109	$ (172,041)
Balance, shares at Dec. 31, 2012		48,148				(16,817)
Net income	39,101				39,101
Unrealized gain on investment securities, net of tax	222			222
Issuance of shares pursuant to stock compensation plan	18,289		(13,742)			$ 32,031
Issuance of shares pursuant to stock compensation plan, shares						1,443
Stock compensation	11,653		11,653
Issuance of shares pursuant to employee stock purchase plan	$ 3,542		2,034			$ 1,508
Issuance of shares pursuant to employee stock purchase plan, shares	64					64
Federal income tax benefit related to exercise of stock options	$ 28,213		28,213
Balance at Dec. 31, 2013	246,319	$ 481	182,176	(46)	202,210	$ (138,502)
Balance, shares at Dec. 31, 2013		48,148				(15,310)
Net income	58,940				58,940
Issuance of shares pursuant to stock compensation plan	14,680		(17,449)			$ 32,129
Issuance of shares pursuant to stock compensation plan, shares						855
Stock compensation	14,819		14,819
Issuance of shares pursuant to employee stock purchase plan	$ 4,144		2,235			$ 1,909
Issuance of shares pursuant to employee stock purchase plan, shares	53					53
Federal income tax benefit related to exercise of stock options	$ 19,415		19,415
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)
Treasury stock purchases	$ (22,817)					$ (22,817)
Treasury stock purchases, shares	(294)					(294)
Issuance of shares for acquisitions	$ 1,473		193			$ 1,280
Issuance of shares for acquisitions, shares	17					17
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
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
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)
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