Consolidated Statements of Shareholders' Equity (USD $) 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, 2011	$ 78,110	$ 481	$ 152,859	$ (355)	$ 130,115	$ (204,990)
Balance, shares at Dec. 31, 2011		48,148				(18,176)
Net income	32,994				32,994
Unrealized gain on investment securities, net of tax	87			87
Issuance of shares pursuant to stock compensation plan	12,443		(17,018)			29,461
Issuance of shares pursuant to stock compensation plan, shares						1,218
Stock compensation	7,411		7,411
Issuance of shares pursuant to employee stock purchase plan	2,641		639			2,002
Issuance of shares pursuant to employee stock purchase plan, shares	81					81
Federal income tax benefit related to exercise of stock options	8,798		8,798
Issuance of shares for acquisition	2,815		1,329			1,486
Issuance of shares for acquisition, shares	60					60
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
Issuance of shares for acquisition	1,473		193			1,280
Issuance of shares for acquisition, shares	17					17
Treasury stock purchases	(22,817)					(22,817)
Treasury stock purchases, shares	(294)					(294)
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)