Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Adjustment to adopt credit-loss guidance	[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, net of taxes	Accumulated Other Comprehensive Income, net of taxes Balance, January 1, 2020	Retained Earnings	Retained Earnings Adjustment to adopt credit-loss guidance	[1]	Retained Earnings Balance, January 1, 2020	Treasury Stock	Treasury Stock Balance, January 1, 2020
Balance (in shares) at Dec. 31, 2019					75,557	75,557
Beginning 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	20,190										20,190
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(533)								(533)
Comprehensive income for the period	19,657
Exercise of stock options and other share-based compensation, net of shares tendered for payment (in shares)					162
Exercise of stock options and other share-based compensation, net of shares tendered for payment	1,724				$ 1		1,723
Payment of statutory withholding on issuance of restricted stock units	(661)						(661)
Share-based compensation expense	1,822						1,822
Treasury shares issued for Deferred Compensation Plan funding and redemptions	528						417								111
Shares issued pursuant to Employee Stock Plan	1,835						1,329								506
Dividends paid and accrued	(15,142)										(15,142)
Shares issued pursuant to Dividend Reinvestment Plan	23						16								7
Other (in shares)					6
Other	187						187
Balance (in shares) at Mar. 31, 2020					75,725
Ending balance at Mar. 31, 2020	$ 438,179				$ 757		275,447		2,386		168,404				(8,815)
Comprehensive income:
Cash dividend paid per common share (in dollars per share)	$ 0.20
Balance (in shares) at Dec. 31, 2019					75,557	75,557
Beginning 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	70,935
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	1,996
Comprehensive income for the period	72,931
Balance (in shares) at Sep. 30, 2020					75,734
Ending balance at Sep. 30, 2020	464,750				$ 757		279,408		4,915		188,578				(8,908)
Balance (in shares) at Mar. 31, 2020					75,725
Beginning balance at Mar. 31, 2020	438,179				$ 757		275,447		2,386		168,404				(8,815)
Comprehensive income:
Net income	23,101										23,101
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	1,656								1,656
Comprehensive income for the period	24,757
Exercise of stock options and other share-based compensation, net of shares tendered for payment (in shares)					7
Exercise of stock options and other share-based compensation, net of shares tendered for payment	138						138
Share-based compensation expense	1,830						1,830
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(2)						51								(53)
Dividends paid and accrued	(15,237)										(15,237)
Shares issued pursuant to Dividend Reinvestment Plan	22						16								6
Balance (in shares) at Jun. 30, 2020					75,732
Ending balance at Jun. 30, 2020	$ 449,687				$ 757		277,482		4,042		176,268				(8,862)
Comprehensive income:
Cash dividend paid per common share (in dollars per share)	$ 0.20
Net income	$ 27,644										27,644
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	873								873
Comprehensive income for the period	28,517
Exercise of stock options and other share-based compensation, net of shares tendered for payment (in shares)					2
Exercise of stock options and other share-based compensation, net of shares tendered for payment	31						31
Payment of statutory withholding on issuance of restricted stock units	(14)						(14)
Share-based compensation expense	1,840						1,840
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(1)						53								(54)
Dividends paid and accrued	(15,334)										(15,334)
Shares issued pursuant to Dividend Reinvestment Plan	24						16								8
Balance (in shares) at Sep. 30, 2020					75,734
Ending balance at Sep. 30, 2020	$ 464,750				$ 757		279,408		4,915		188,578				(8,908)
Comprehensive income:
Cash dividend paid per common share (in dollars per share)	$ 0.20
Balance (in shares) at Dec. 31, 2020					75,798
Beginning balance at Dec. 31, 2020	$ 480,461				$ 758		282,206		5,563		200,893				(8,959)
Comprehensive income:
Net income	24,653										24,653
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(1,271)								(1,271)
Comprehensive income for the period	23,382
Exercise of stock options and other share-based compensation, net of shares tendered for payment (in shares)					156
Exercise of stock options and other share-based compensation, net of shares tendered for payment	1,475				$ 2		1,473
Payment of statutory withholding on issuance of restricted stock units	(1,501)						(1,501)
Share-based compensation expense	2,104						2,104
Treasury shares issued for Deferred Compensation Plan funding and redemptions	509						545								(36)
Shares issued pursuant to Employee Stock Plan	2,052						1,554								498
Dividends paid and accrued	(15,526)										(15,526)
Shares issued pursuant to Dividend Reinvestment Plan	23						18								5
Other (in shares)					4
Other	123						123
Balance (in shares) at Mar. 31, 2021					75,958
Ending balance at Mar. 31, 2021	$ 493,102				$ 760		286,522		4,292		210,020				(8,492)
Comprehensive income:
Cash dividend paid per common share (in dollars per share)	$ 0.21
Balance (in shares) at Dec. 31, 2020					75,798
Beginning balance at Dec. 31, 2020	$ 480,461				$ 758		282,206		5,563		200,893				(8,959)
Comprehensive income:
Net income	43,758
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(1,582)
Comprehensive income for the period	42,176
Balance (in shares) at Sep. 30, 2021					75,997
Ending balance at Sep. 30, 2021	480,205				$ 760		291,899		3,981		197,550				(13,985)
Balance (in shares) at Mar. 31, 2021					75,958
Beginning balance at Mar. 31, 2021	493,102				$ 760		286,522		4,292		210,020				(8,492)
Comprehensive income:
Net income	9,565										9,565
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	703								703
Comprehensive income for the period	10,268
Exercise of stock options and other share-based compensation, net of shares tendered for payment (in shares)					11
Exercise of stock options and other share-based compensation, net of shares tendered for payment	380						380
Share-based compensation expense	2,159						2,159
Purchases of treasury stock	(1,835)														(1,835)
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(1)						60								(61)
Dividends paid and accrued	(15,792)										(15,792)
Shares issued pursuant to Dividend Reinvestment Plan	23						17								6
Balance (in shares) at Jun. 30, 2021					75,969
Ending balance at Jun. 30, 2021	$ 488,304				$ 760		289,138		4,995		203,793				(10,382)
Comprehensive income:
Cash dividend paid per common share (in dollars per share)	$ 0.21
Net income	$ 9,540										9,540
Unrealized (loss) gain on available-for-sale marketable securities, net of taxes	(1,014)								(1,014)
Comprehensive income for the period	8,526
Exercise of stock options and other share-based compensation, net of shares tendered for payment (in shares)					28
Exercise of stock options and other share-based compensation, net of shares tendered for payment	383						383
Payment of statutory withholding on issuance of restricted stock units	100						100
Share-based compensation expense	2,214						2,214
Purchases of treasury stock	(3,562)														(3,562)
Treasury shares issued for Deferred Compensation Plan funding and redemptions	0						50								(50)
Dividends paid and accrued	(15,783)										(15,783)
Shares issued pursuant to Dividend Reinvestment Plan	23						14								9
Balance (in shares) at Sep. 30, 2021					75,997
Ending balance at Sep. 30, 2021	$ 480,205				$ 760		$ 291,899		$ 3,981		$ 197,550				$ (13,985)
Comprehensive income:
Cash dividend paid per common share (in dollars per share)	$ 0.21

[1] See Note 4—Allowance for Doubtful Accounts regarding the new credit-loss guidance.