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
Beginning balance (in shares) at Dec. 31, 2016		74,204
Beginning 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, 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 Plan	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
Ending balance (in shares) at Dec. 31, 2017		74,960
Ending 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 gain (loss) 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 Plan	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
Ending balance (in shares) at Dec. 31, 2018		75,344
Ending balance at Dec. 31, 2018	440,780	$ 753	259,440	158	190,092	(9,663)
Comprehensive income:
Net income for the period	64,581				64,581
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	2,761			2,761
Total comprehensive income	67,342
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		207
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	3,628	$ 3	3,625
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(587)		(587)
Share-based compensation expense — stock options, restricted stock and restricted stock units	6,590		6,590
Treasury shares issued for Deferred Compensation Plan funding and redemptions	388		535			(147)
Shares issued pursuant to Employee Stock Plan	2,130		1,781			349
Dividends paid and accrued	(59,218)				(59,218)
Shares issued pursuant to Dividend Reinvestment Plan	90		68			22
Contingent shares settled pursuant to acquisition	(1,012)		(1,012)
Other (in shares)		6
Other	174		174
Ending balance (in shares) at Dec. 31, 2019		75,557
Ending balance at Dec. 31, 2019	$ 460,305	$ 756	$ 270,614	$ 2,919	$ 195,455	$ (9,439)