Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Adjustment to adopt credit-loss guidance1	[1]	Balance, January 1, 2020	Common Stock	Common Stock Balance, January 1, 2020	Additional Paid-in Capital	Additional Paid-in Capital Balance, January 1, 2020	Accumulated Other Comprehensive Income (Loss), net of Taxes	Accumulated Other Comprehensive Income (Loss), net of Taxes Balance, January 1, 2020	Retained Earnings	Retained Earnings Adjustment to adopt credit-loss guidance1	[1]	Retained Earnings Balance, January 1, 2020	Treasury Stock	Treasury Stock Balance, January 1, 2020
Balance (in shares) at Dec. 31, 2018					75,344
Beginning 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 on available-for-sale marketable securities, net of taxes	2,761								2,761
Comprehensive income for the period	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 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:
Net income for the period	98,682										98,682
Unrealized gain on available-for-sale marketable securities, net of taxes	2,644								2,644
Comprehensive income for the period	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 units	(675)						(675)
Share-based expense	7,329						7,329
Purchases of treasury stock	0
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)
Comprehensive income:
Net income for the period	45,863										45,863
Unrealized gain on available-for-sale marketable securities, net of taxes	(1,563)								(1,563)
Comprehensive income for the period	44,300
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	2,425				$ 2		2,423
Payment of statutory withholding on issuance of restricted stock units	(1,410)						(1,410)
Share-based expense	8,600						8,600
Purchases of treasury stock	(21,535)														(21,535)
Treasury shares issued for Deferred Compensation Plan funding	368						574								(206)
Shares issued pursuant to Employee Stock Plan	2,052						1,554								498
Dividends paid and accrued	(62,799)										(62,799)
Shares issued pursuant to Dividend Reinvestment Plan	92						54								38
Other (in shares)					4
Other	123						123
Balance (in shares) at Dec. 31, 2021					76,009
Ending balance at Dec. 31, 2021	$ 452,677				$ 760		$ 294,124		$ 4,000		$ 183,957				$ (30,164)

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