Consolidated Statements Of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Retained Earnings	Treasury Stock
Balance (in shares) at Dec. 31, 2012		70,036
Balance at Dec. 31, 2012	$ 229,570	$ 700	$ 113,495	$ 127	$ 127,042	$ (11,794)
Comprehensive income:
Net income for the period	47,129				47,129
Unrealized gain (loss) on available for sale marketable securities, net of taxes	(78)			(78)
Total comprehensive income	47,051
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		617
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	6,428	$ 7	6,381			40
Tax benefit from equity compensation plans	2,615		2,615
Share-based compensation expense — stock options and restricted stock	2,045		2,045
Treasury shares issued for Deferred Compensation Plan funding and redemptions	360		294			66
Shares issued pursuant to Employee Stock Plans	1,842		1,370			472
Cash dividends	(46,707)				(46,707)
Shares issued pursuant to Dividend Reinvestment Plan	107		309			(202)
Shares issued pursuant to acquisition	41,832		41,820			0
Balance (in shares) at Dec. 31, 2013		71,868
Balance at Dec. 31, 2013	285,143	$ 719	168,329	49	127,464	$ (11,418)
Comprehensive income:
Net income for the period	21,850				21,850
Unrealized gain (loss) on available for sale marketable securities, net of taxes	(24)			(24)
Total comprehensive income	21,826
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		534
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	6,196	$ 5	6,191
Tax benefit from equity compensation plans	2,626		2,626
Share-based compensation expense — stock options and restricted stock	2,705		2,705
Treasury shares issued for Deferred Compensation Plan funding and redemptions	516		459			$ 57
Shares issued pursuant to Employee Stock Plans	1,851		1,457			394
Cash dividends	(49,077)				(49,077)
Shares issued pursuant to Dividend Reinvestment Plan	110		326			$ (216)
Shares issued pursuant to acquisition (in shares)		476
Shares issued pursuant to acquisition	0	$ 5	(5)
Balance (in shares) at Dec. 31, 2014		72,878
Balance at Dec. 31, 2014	275,830	$ 729	186,022	25	100,237	$ (11,183)
Comprehensive income:
Net income for the period	58,024				58,024
Unrealized gain (loss) on available for sale marketable securities, net of taxes	518			518
Total comprehensive income	58,542
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		386
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	6,251	$ 4	6,247
Tax benefit from equity compensation plans	1,873		1,873
Share-based compensation expense — stock options and restricted stock	3,033		3,033
Treasury shares issued for Deferred Compensation Plan funding and redemptions	488		418			$ 70
Shares issued pursuant to Employee Stock Plans	1,701		1,363			338
Cash dividends	(51,375)				(51,375)
Shares issued pursuant to Dividend Reinvestment Plan	113		343			$ (230)
Shares issued pursuant to acquisition (in shares)		529
Shares issued pursuant to acquisition	0	$ 5	(5)
Balance (in shares) at Dec. 31, 2015		73,793
Balance at Dec. 31, 2015	$ 296,456	$ 738	$ 199,294	$ 543	$ 106,886	$ (11,005)