Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income, net of taxes	Retained Earnings	Treasury Stock
Balance (in shares) at Dec. 31, 2017		74,960,000
Beginning balance at Dec. 31, 2017	$ 399,952	$ 750	$ 244,363	$ 837	$ 163,860	$ (9,858)
Comprehensive income:
Net income for the period	72				72
Unrealized gain on available-for-sale marketable securities, net of taxes	(1,138)			(1,138)
Total comprehensive income	(1,066)
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		205,000
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	5,005	$ 2	5,003
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(295)		(295)
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,408		1,408
Treasury shares issued for Deferred Compensation Plan funding and redemptions	480		514			(34)
Shares issued pursuant to Employee Stock Plan	2,820		2,474			346
Dividends paid and accrued, $0.20 per share	(14,189)				(14,189)
Shares issued pursuant to Dividend Reinvestment Plan	24		20			4
Other (in shares)		4,000
Other	205		205
Balance (in shares) at Mar. 31, 2018		75,169,000
Ending balance at Mar. 31, 2018	394,344	$ 752	253,692	(301)	149,743	(9,542)
Balance (in shares) at Dec. 31, 2017		74,960,000
Beginning balance at Dec. 31, 2017	399,952	$ 750	244,363	837	163,860	(9,858)
Comprehensive income:
Net income for the period	51,972
Unrealized gain on available-for-sale marketable securities, net of taxes	(1,549)
Total comprehensive income	50,423
Balance (in shares) at Sep. 30, 2018		75,256,000
Ending balance at Sep. 30, 2018	419,580	$ 753	256,129	(712)	173,033	(9,623)
Balance (in shares) at Mar. 31, 2018		75,169,000
Beginning balance at Mar. 31, 2018	394,344	$ 752	253,692	(301)	149,743	(9,542)
Comprehensive income:
Net income for the period	25,814				25,814
Unrealized gain on available-for-sale marketable securities, net of taxes	97			97
Total comprehensive income	25,911
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		31,000
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	750		750
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,398		1,398
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(3)		35			(38)
Dividends paid and accrued, $0.20 per share	(14,288)				(14,288)
Shares issued pursuant to Dividend Reinvestment Plan	22		19			3
Balance (in shares) at Jun. 30, 2018		75,200,000
Ending balance at Jun. 30, 2018	408,134	$ 752	255,894	(204)	161,269	(9,577)
Comprehensive income:
Net income for the period	26,086				26,086
Unrealized gain on available-for-sale marketable securities, net of taxes	(508)			(508)
Total comprehensive income	25,578
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		56,000
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	1,111	$ 1	1,110
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(17)		(17)
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,368		1,368
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(2)		48			(50)
Shares issued pursuant to Employee Stock Plan	(1)		(1)
Dividends paid and accrued, $0.20 per share	(14,391)				(14,391)
Shares issued pursuant to Dividend Reinvestment Plan	22		18			4
Contingent shares settled pursuant to acquisition	(2,291)		(2,291)
Other (in shares)		0
Other	69				69
Balance (in shares) at Sep. 30, 2018		75,256,000
Ending balance at Sep. 30, 2018	419,580	$ 753	256,129	(712)	173,033	(9,623)
Balance (in shares) at Dec. 31, 2018		75,344,000
Beginning balance at Dec. 31, 2018	440,780	$ 753	259,440	158	190,092	(9,663)
Comprehensive income:
Net income for the period	9,156				9,156
Unrealized gain on available-for-sale marketable securities, net of taxes	1,416			1,416
Total comprehensive income	10,572
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		115,000
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	1,913	$ 2	1,911
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(579)		(579)
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,656		1,656
Treasury shares issued for Deferred Compensation Plan funding and redemptions	536		535			1
Shares issued pursuant to Employee Stock Plan	2,130		1,781			349
Dividends paid and accrued, $0.20 per share	(14,656)				(14,656)
Shares issued pursuant to Dividend Reinvestment Plan	23		18			5
Other (in shares)		6,000
Other	174		174
Balance (in shares) at Mar. 31, 2019		75,465,000
Ending balance at Mar. 31, 2019	442,549	$ 755	264,936	1,574	184,592	(9,308)
Balance (in shares) at Dec. 31, 2018		75,344,000
Beginning balance at Dec. 31, 2018	440,780	$ 753	259,440	158	190,092	(9,663)
Comprehensive income:
Net income for the period	45,686
Unrealized gain on available-for-sale marketable securities, net of taxes	2,664
Total comprehensive income	48,350
Balance (in shares) at Sep. 30, 2019		75,524,000
Ending balance at Sep. 30, 2019	455,299	$ 755	269,598	2,822	191,515	(9,391)
Balance (in shares) at Mar. 31, 2019		75,465,000
Beginning balance at Mar. 31, 2019	442,549	$ 755	264,936	1,574	184,592	(9,308)
Comprehensive income:
Net income for the period	18,186				18,186
Unrealized gain on available-for-sale marketable securities, net of taxes	791			791
Total comprehensive income	18,977
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		54,000
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	1,186		1,186
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,668		1,668
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(2)		45			(47)
Dividends paid and accrued, $0.20 per share	(14,753)				(14,753)
Shares issued pursuant to Dividend Reinvestment Plan	23		18			5
Balance (in shares) at Jun. 30, 2019		75,519,000
Ending balance at Jun. 30, 2019	449,648	$ 755	267,853	2,365	188,025	(9,350)
Comprehensive income:
Net income for the period	18,344				18,344
Unrealized gain on available-for-sale marketable securities, net of taxes	457			457
Total comprehensive income	18,801
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		5,000
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	67		67
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(8)		(8)
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,626		1,626
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(2)		45			(47)
Dividends paid and accrued, $0.20 per share	(14,854)				(14,854)
Shares issued pursuant to Dividend Reinvestment Plan	21		15			6
Balance (in shares) at Sep. 30, 2019		75,524,000
Ending balance at Sep. 30, 2019	$ 455,299	$ 755	$ 269,598	$ 2,822	$ 191,515	$ (9,391)