CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - UNAUDITED - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Common Stock Warrants	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Accumulated Earnings
Balance, beginning of period at Dec. 31, 2018	$ 361,024	$ 693	$ 9,082	$ 441,417	$ (842,360)	$ (42,935)	$ 795,127
Balance, beginning of period, shares at Dec. 31, 2018		33,158
Net income							19,251
Unrealized gain on short-term investments, net of tax						18
Foreign currency translation adjustments						3,847
Total comprehensive income, net of tax	23,116
Repurchase of common stock	(13,424)			(4,134)	(9,290)
Repurchase of common stock, shares		(352)
Issuance of common stock pursuant to employee stock purchase plan	512			512
Issuance of common stock pursuant to employee stock purchase plan, shares		15
Issuance of restricted common stock pursuant to stock-based compensation plans		$ 4		(4)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		462
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(3)
Stock-based compensation expense	3,693			3,693
Declaration of cash dividends	(7,411)						(7,411)
Balance, ending of period at Mar. 31, 2019	367,510	$ 697	9,082	441,484	(851,650)	(39,070)	806,967
Balance, ending of period, shares at Mar. 31, 2019		33,280
Balance, beginning of period at Dec. 31, 2018	361,024	$ 693	9,082	441,417	(842,360)	(42,935)	795,127
Balance, beginning of period, shares at Dec. 31, 2018		33,158
Net income	60,189
Unrealized gain on short-term investments, net of tax	24
Foreign currency translation adjustments	(6,868)
Total comprehensive income, net of tax	53,345
Balance, ending of period at Sep. 30, 2019	382,036	$ 696	9,082	451,606	(862,917)	(49,779)	833,348
Balance, ending of period, shares at Sep. 30, 2019		32,979
Balance, beginning of period at Mar. 31, 2019	367,510	$ 697	9,082	441,484	(851,650)	(39,070)	806,967
Balance, beginning of period, shares at Mar. 31, 2019		33,280
Net income							19,379
Unrealized gain on short-term investments, net of tax						10
Foreign currency translation adjustments						(3,818)
Total comprehensive income, net of tax	15,571
Repurchase of common stock	(6,919)			(383)	(6,536)
Repurchase of common stock, shares		(148)
Issuance of common stock pursuant to employee stock purchase plan	603			603
Issuance of common stock pursuant to employee stock purchase plan, shares		15
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		$ (1)		1
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(91)
Stock-based compensation expense	4,807			4,807
Declaration of cash dividends	(7,248)						(7,248)
Balance, ending of period at Jun. 30, 2019	374,324	$ 696	9,082	446,512	(858,186)	(42,878)	819,098
Balance, ending of period, shares at Jun. 30, 2019		33,062
Net income	21,559						21,559
Unrealized gain on short-term investments, net of tax	(4)					(4)
Foreign currency translation adjustments	(6,897)					(6,897)
Total comprehensive income, net of tax	14,658
Repurchase of common stock	(4,947)			(216)	(4,731)
Repurchase of common stock, shares		(97)
Issuance of common stock pursuant to employee stock purchase plan	515			515
Issuance of common stock pursuant to employee stock purchase plan, shares		12
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		34
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(32)
Stock-based compensation expense	4,795			4,795
Declaration of cash dividends	(7,309)						(7,309)
Balance, ending of period at Sep. 30, 2019	382,036	$ 696	$ 9,082	451,606	(862,917)	(49,779)	833,348
Balance, ending of period, shares at Sep. 30, 2019		32,979
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	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
Declaration of cash 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	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	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
Declaration of cash 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
Declaration of cash 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	33,125