CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - UNAUDITED - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Accumulated Earnings	Noncontrolling Interest
Balance, beginning of period at Dec. 31, 2019	$ 396,662	$ 696	$ 454,663	$ (867,817)	$ (39,503)	$ 848,623
Balance, beginning of period, shares at Dec. 31, 2019		32,891
Net income						21,514
Unrealized gain on short-term investments, net of tax					(24)
Foreign currency translation adjustments					(15,084)
Total comprehensive income, net of tax	6,406
Repurchase of common stock	(13,965)	$ (2)	(7,555)	(6,408)
Repurchase of common stock, shares		(299)
Issuance of common stock pursuant to employee stock purchase plan	564		564
Issuance of common stock pursuant to employee stock purchase plan, shares		14
Issuance of restricted common stock pursuant to stock-based compensation plans		$ 5	(5)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		476
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(7)
Stock-based compensation expense	4,857		4,857
Dividends	(7,693)					(7,693)
Balance, ending of period at Mar. 31, 2020	386,831	$ 699	452,524	(874,225)	(54,611)	862,444
Balance, ending of period, shares at Mar. 31, 2020		33,075
Balance, beginning of period at Dec. 31, 2019	396,662	$ 696	454,663	(867,817)	(39,503)	848,623
Balance, beginning of period, shares at Dec. 31, 2019		32,891
Net income	45,446
Unrealized gain on short-term investments, net of tax	14
Foreign currency translation adjustments	(4,746)
Total comprehensive income, net of tax	40,714
Balance, ending of period at Sep. 30, 2020	409,927	$ 701	462,775	(880,162)	(44,235)	870,848
Balance, ending of period, shares at Sep. 30, 2020		33,125
Balance, beginning of period at Mar. 31, 2020	386,831	$ 699	452,524	(874,225)	(54,611)	862,444
Balance, beginning of period, shares at Mar. 31, 2020		33,075
Net income						10,366
Unrealized gain on short-term investments, net of tax					52
Foreign currency translation adjustments					2,671
Total comprehensive income, net of tax	13,089
Repurchase of common stock	(467)		(100)	(367)
Repurchase of common stock, shares		(11)
Issuance of common stock pursuant to employee stock purchase plan	683		683
Issuance of common stock pursuant to employee stock purchase plan, shares		18
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		12
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(14)
Stock-based compensation expense	5,255		5,255
Dividends	(7,769)					(7,769)
Balance, ending of period at Jun. 30, 2020	397,622	$ 699	458,362	(874,592)	(51,888)	865,041
Balance, ending of period, shares at Jun. 30, 2020		33,080
Net income	13,566					13,566
Unrealized gain on short-term investments, net of tax	(14)				(14)
Foreign currency translation adjustments	7,667				7,667
Total comprehensive income, net of tax	21,219
Repurchase of common stock	(5,722)		(152)	(5,570)
Repurchase of common stock, shares		(143)
Issuance of common stock pursuant to employee stock purchase plan	591		591
Issuance of common stock pursuant to employee stock purchase plan, shares		16
Issuance of restricted common stock pursuant to stock-based compensation plans		$ 2	(2)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		183
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(11)
Stock-based compensation expense	3,976		3,976
Dividends	(7,759)					(7,759)
Balance, ending of period at Sep. 30, 2020	409,927	$ 701	462,775	(880,162)	(44,235)	870,848
Balance, ending of period, shares at Sep. 30, 2020		33,125
Balance, beginning of period at Dec. 31, 2020	$ 422,395	$ 700	470,557	(894,126)	(31,138)	876,402
Balance, beginning of period, shares at Dec. 31, 2020	32,713	32,713
Net income						19,631
Unrealized gain on short-term investments, net of tax					(6)
Foreign currency translation adjustments					(355)
Total comprehensive income, net of tax	$ 19,270
Repurchase of common stock	(11,721)	$ (1)	(5,202)	(6,518)
Repurchase of common stock, shares		(252)
Issuance of common stock pursuant to employee stock purchase plan	619		619
Issuance of common stock pursuant to employee stock purchase plan, shares		16
Issuance of restricted common stock pursuant to stock-based compensation plans		$ 5	(5)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		487
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(1)
Stock-based compensation expense	5,395		5,395
Dividends	(8,243)					(8,243)
Balance, ending of period at Mar. 31, 2021	427,715	$ 704	471,364	(900,644)	(31,499)	887,790
Balance, ending of period, shares at Mar. 31, 2021		32,963
Balance, beginning of period at Dec. 31, 2020	$ 422,395	$ 700	470,557	(894,126)	(31,138)	876,402
Balance, beginning of period, shares at Dec. 31, 2020	32,713	32,713
Net income	$ 55,082
Unrealized gain on short-term investments, net of tax	(10)
Foreign currency translation adjustments	(4,635)
Total comprehensive income, net of tax	50,437
Balance, ending of period at Sep. 30, 2021	$ 443,624	$ 704	482,387	(914,274)	(35,783)	906,955	$ 3,635
Balance, ending of period, shares at Sep. 30, 2021	32,687	32,687
Balance, beginning of period at Mar. 31, 2021	$ 427,715	$ 704	471,364	(900,644)	(31,499)	887,790
Balance, beginning of period, shares at Mar. 31, 2021		32,963
Net income						19,321
Unrealized gain on short-term investments, net of tax					(7)
Foreign currency translation adjustments					2,212
Total comprehensive income, net of tax	21,526
Repurchase of common stock	(7,049)		(92)	(6,957)
Repurchase of common stock, shares		(156)
Issuance of common stock pursuant to employee stock purchase plan, shares		19
Issuance of restricted common stock pursuant to stock-based compensation plans	716		716
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		6
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(35)
Stock-based compensation expense	5,022		5,022
Dividends	(8,150)					(8,150)
Balance, ending of period at Jun. 30, 2021	439,780	$ 704	477,010	(907,601)	(29,294)	898,961
Balance, ending of period, shares at Jun. 30, 2021		32,797
Net income	16,130					16,130
Unrealized gain on short-term investments, net of tax	3				3
Foreign currency translation adjustments	(6,492)				(6,492)
Total comprehensive income, net of tax	9,641
Repurchase of common stock	(6,774)		(101)	(6,673)
Repurchase of common stock, shares		(145)
Issuance of common stock pursuant to employee stock purchase plan	591		591
Issuance of common stock pursuant to employee stock purchase plan, shares		15
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		56
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(36)
Stock-based compensation expense	4,887		4,887
Dividends	(8,136)					(8,136)
Noncontrolling interest related to business combination	3,635						3,635
Balance, ending of period at Sep. 30, 2021	$ 443,624	$ 704	$ 482,387	$ (914,274)	$ (35,783)	$ 906,955	$ 3,635
Balance, ending of period, shares at Sep. 30, 2021	32,687	32,687