Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Adjustment to adopt credit-loss guidance	[1]	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Income, net of taxes	Accumulated Other Comprehensive Income, net of taxes Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Adjustment to adopt credit-loss guidance	[1]	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance
Balance (in shares) at Dec. 31, 2017					74,960
Beginning 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 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 share-based compensation, net of shares tendered for payment	8,517				$ 3		8,514
Share-based expense	5,580						5,580
Treasury shares issued for Deferred Compensation Plan funding	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
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 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 share-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 expense	6,590						6,590
Treasury shares issued for Deferred Compensation Plan funding	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
Balance (in shares) at Dec. 31, 2019					75,557	75,557
Ending balance at Dec. 31, 2019	$ 460,305	$ (32,099)		$ 428,206	$ 756	$ 756	270,614	$ 270,614	2,919	$ 2,919	195,455	$ (32,099)		$ 163,356	(9,439)	$ (9,439)
Comprehensive income:
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Net income for the period	$ 98,682										98,682
Unrealized loss on available-for-sale marketable securities, net of taxes	2,644								2,644
Total comprehensive income	101,326
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)					235
Exercise of stock options and other share-based compensation, net of shares tendered for payment	2,920				$ 2		2,918
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(675)						(675)
Share-based expense	7,329						7,329
Treasury shares issued for Deferred Compensation Plan funding	385						438								(53)
Shares issued pursuant to Employee Stock Plan	1,835						1,329								506
Dividends paid and accrued	(61,145)										(61,145)
Shares issued pursuant to Dividend Reinvestment Plan	93						66								27
Other (in shares)					6
Other	187						187
Balance (in shares) at Dec. 31, 2020					75,798
Ending balance at Dec. 31, 2020	$ 480,461				$ 758		$ 282,206		$ 5,563		$ 200,893				$ (8,959)

[1] See Note 8 — Allowance for Doubtful Accounts herein regarding the new credit-loss guidance