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, 2017	$ 342,746	$ 689	$ 9,082	$ 427,091	$ (814,732)	$ (28,822)	$ 749,438
Balance, beginning of period, shares at Dec. 31, 2017		33,516
Net income							14,014
Unrealized gain (loss) on short-term investments, net of tax						(94)
Foreign currency translation adjustments						7,710
Total comprehensive income, net of tax	21,630
Repurchase of common stock	(11,920)			(6,218)	(5,702)
Repurchase of common stock, shares		(255)
Issuance of common stock pursuant to employee stock purchase plan	484			484
Issuance of common stock pursuant to employee stock purchase plan, shares		14
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		458
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		(59)
Stock-based compensation expense	4,572			4,572
Declaration of cash dividends	(6,999)						(6,999)
Adjustments due to adoption of new accounting standards	7,562						7,562
Balance, ending of period at Mar. 31, 2018	358,075	$ 692	9,082	425,926	(820,434)	(21,206)	764,015
Balance, ending of period, shares at Mar. 31, 2018		33,674
Balance, beginning of period at Dec. 31, 2017	342,746	$ 689	9,082	427,091	(814,732)	(28,822)	749,438
Balance, beginning of period, shares at Dec. 31, 2017		33,516
Net income	45,232
Unrealized gain (loss) on short-term investments, net of tax	81
Foreign currency translation adjustments	(8,630)
Total comprehensive income, net of tax	36,683
Balance, ending of period at Sep. 30, 2018	358,419	$ 693	9,082	436,412	(831,585)	(37,371)	781,188
Balance, ending of period, shares at Sep. 30, 2018		33,451
Balance, beginning of period at Mar. 31, 2018	358,075	$ 692	9,082	425,926	(820,434)	(21,206)	764,015
Balance, beginning of period, shares at Mar. 31, 2018		33,674
Net income							15,117
Unrealized gain (loss) on short-term investments, net of tax						68
Foreign currency translation adjustments						(16,231)
Total comprehensive income, net of tax	(1,046)
Repurchase of common stock	(6,399)			(767)	(5,632)
Repurchase of common stock, shares		(153)
Issuance of common stock pursuant to employee stock purchase plan	650			650
Issuance of common stock pursuant to employee stock purchase plan, shares		18
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		41
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(19)
Stock-based compensation expense	5,641			5,641
Declaration of cash dividends	(7,061)						(7,061)
Balance, ending of period at Jun. 30, 2018	349,860	$ 692	9,082	431,450	(826,066)	(37,369)	772,071
Balance, ending of period, shares at Jun. 30, 2018		33,561
Net income	16,101						16,101
Unrealized gain (loss) on short-term investments, net of tax	107					107
Foreign currency translation adjustments	(109)					(109)
Total comprehensive income, net of tax	16,099
Repurchase of common stock	(5,715)			(196)	(5,519)
Repurchase of common stock, shares		(141)
Issuance of common stock pursuant to employee stock purchase plan	567			567
Issuance of common stock pursuant to employee stock purchase plan, shares		17
Issuance of restricted common stock pursuant to stock-based compensation plans		$ 1		(1)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		33
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(19)
Stock-based compensation expense	4,592			4,592
Declaration of cash dividends	(6,984)						(6,984)
Balance, ending of period at Sep. 30, 2018	358,419	$ 693	9,082	436,412	(831,585)	(37,371)	781,188
Balance, ending of period, shares at Sep. 30, 2018		33,451
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	33,158
Net income							19,251
Unrealized gain (loss) 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	33,158
Net income	$ 60,189
Unrealized gain (loss) 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	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 (loss) 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 (loss) 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	32,979