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 gain (loss) 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)
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	43,291
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	1,123
Comprehensive income for the period	44,414
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)
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 gain (loss) 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)
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 gain (loss) 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)
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	34,218
Unrealized gain (loss) on available-for-sale marketable securities, net of taxes	(568)
Comprehensive income for the period	33,650
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)
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 gain (loss) 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)

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