Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss), net of Taxes	Retained Earnings	Treasury Stock
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
Dividends paid	(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)
Comprehensive income:
Net income for the period	77,396				77,396
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	(862)			(862)
Total comprehensive income	76,534
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		301
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	5,968	$ 3	5,965
Tax benefit from equity compensation plans	2,773		2,773
Share-based compensation expense — stock options and restricted stock	3,743		3,743
Treasury shares issued for Deferred Compensation Plan funding and redemptions	534		103			431
Shares issued pursuant to Employee Stock Plans	2,067		1,696			371
Dividends paid	(53,342)				(53,342)
Shares issued pursuant to Dividend Reinvestment Plan	109		91			18
Shares issued pursuant to previous settlement (in shares)		113
Shares issued pursuant to previous settlement	4,000	$ 1	3,999
Other (in shares)		(3)
Other	0
Balance (in shares) at Dec. 31, 2016		74,204
Balance at Dec. 31, 2016	338,842	$ 742	217,664	(319)	130,940	(10,185)
Comprehensive income:
Net income for the period	88,226				88,226
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	1,156			1,156
Total comprehensive income	89,382
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		697
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	12,808	$ 7	12,801
Share-based compensation expense — stock options and restricted stock	4,945		4,945
Treasury shares issued for Deferred Compensation Plan funding and redemptions	156		181			(25)
Shares issued pursuant to Employee Stock Plans	2,091		1,752			339
Dividends paid	(55,306)				(55,306)
Shares issued pursuant to Dividend Reinvestment Plan	95		82			13
Shares issued pursuant to acquisition (in shares)		59
Shares issued pursuant to acquisition	6,939	$ 1	6,938
Balance (in shares) at Dec. 31, 2017		74,960
Balance at Dec. 31, 2017	$ 399,952	$ 750	$ 244,363	$ 837	$ 163,860	$ (9,858)