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, 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 (loss) gain 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, restricted stock and restricted stock units	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 and accrued	(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 (loss) gain 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, restricted stock and restricted stock units	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 and accrued	(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)
Comprehensive income:
Net income for the period	83,524				83,524
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(679)			(679)
Total comprehensive income	82,845
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		380
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	8,517	$ 3	8,514
Share-based compensation expense — stock options, restricted stock and restricted stock units	5,580		5,580
Treasury shares issued for Deferred Compensation Plan funding and redemptions	354		519			(165)
Shares issued pursuant to Employee Stock Plans	2,821		2,475			346
Dividends paid and accrued	(57,361)				(57,361)
Shares issued pursuant to Dividend Reinvestment Plan	89		75			14
Contingent shares settled pursuant to acquisition	(2,291)		(2,291)
Other (in shares)		4
Other	274		205		69
Balance (in shares) at Dec. 31, 2018		75,344
Balance at Dec. 31, 2018	$ 440,780	$ 753	$ 259,440	$ 158	$ 190,092	$ (9,663)