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 for the period	20,190										20,190
Unrealized 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 and 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, $0.21 per share	(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, 2020					75,798
Beginning balance at Dec. 31, 2020	480,461				$ 758		282,206		5,563		200,893				(8,959)
Comprehensive income:
Net income for the period	24,653										24,653
Unrealized 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 and 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, $0.21 per share	(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)

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