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)
Comprehensive income for the period	(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	(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	25,886
Unrealized gain on available-for-sale marketable securities, net of taxes	(1,041)
Comprehensive income for the period	24,845
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)
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
Comprehensive income for the period	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	(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)
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
Comprehensive income for the period	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	(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	27,342
Unrealized gain on available-for-sale marketable securities, net of taxes	2,207
Comprehensive income for the period	29,549
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)
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
Comprehensive income for the period	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	$ 0	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	(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)