Consolidated Statements Of Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income [Member]	Retained Earnings [Member]	Treasury Stock [Member]
Balance at Dec. 31, 2009	$ 208,774	$ 687	$ 92,110	$ 0	$ 135,837	$ (19,860)
Balance, Shares at Dec. 31, 2009		68,729
Comprehensive income:
Net income for the period	34,441				34,441
Unrealized gain/(loss) on available for sale marketable securities, net of taxes	(78)			(78)
Total Comprehensive Income	34,363
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	4,790	6	4,167			617
Exercise of stock options and other stock-based compensation, net of shares tendered for payment, Shares		586
Tax benefit from equity compensation plans	1,938		1,938
Share-based compensation expense - stock options	1,015		1,015
Treasury shares issued for Deferred Compensation Plan funding and redemptions	316		226			90
Shares issued pursuant to Employee Stock Plans	1,047		609			438
Cash dividends	(39,285)				(39,285)
Shares issued pursuant to Dividend Reinvestment Plan	121		73			48
Balance at Dec. 31, 2010	213,079	693	100,138	(78)	130,993	(18,667)
Balance, Shares at Dec. 31, 2010		69,315
Comprehensive income:
Net income for the period	38,156				38,156
Unrealized gain/(loss) on available for sale marketable securities, net of taxes	421			421
Total Comprehensive Income	38,577
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	2,363	2	349			2,012
Exercise of stock options and other stock-based compensation, net of shares tendered for payment, Shares		158
Tax benefit from equity compensation plans	1,222		1,222
Share-based compensation expense - stock options	1,870		1,870
Treasury shares issued for Deferred Compensation Plan funding and redemptions	402		367			35
Shares issued pursuant to Employee Stock Plans	1,233		782			451
Cash dividends	(42,228)				(42,228)
Shares issued pursuant to Dividend Reinvestment Plan	128		81			47
Shares issue pursuant to acquisition	1,080		918			162
Balance at Dec. 31, 2011	217,726	695	105,727	343	126,921	(15,960)
Balance, Shares at Dec. 31, 2011		69,473
Comprehensive income:
Net income for the period	44,214				44,214
Unrealized gain/(loss) on available for sale marketable securities, net of taxes	(216)			(216)
Total Comprehensive Income	43,998
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	5,573	5	4,098			1,470
Exercise of stock options and other stock-based compensation, net of shares tendered for payment, Shares		563
Tax benefit from equity compensation plans	2,649		2,649
Share-based compensation expense - stock options	1,897		1,897
Treasury shares issued for Deferred Compensation Plan funding and redemptions	453		394			59
Shares issued pursuant to Employee Stock Plans	1,249		1,278			(29)
Cash dividends	(44,093)				(44,093)
Shares issued pursuant to Dividend Reinvestment Plan	118		85			33
Shares issue pursuant to acquisition	0		(2,633)			2,633
Balance at Dec. 31, 2012	$ 229,570	$ 700	$ 113,495	$ 127	$ 127,042	$ (11,794)
Balance, Shares at Dec. 31, 2012		70,036