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 (loss), 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	29,131
Unrealized gain (loss) on short-term investments, net of tax	(26)
Foreign currency translation adjustments	(8,521)
Total comprehensive income (loss), net of tax	20,584
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
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						15,117
Unrealized gain (loss) on short-term investments, net of tax	68					68
Foreign currency translation adjustments	(16,231)					(16,231)
Total comprehensive income (loss), 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
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 (loss), 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	$ 38,630
Unrealized gain (loss) on short-term investments, net of tax	28
Foreign currency translation adjustments	29
Total comprehensive income (loss), net of tax	38,687
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	33,062
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						19,379
Unrealized gain (loss) on short-term investments, net of tax	10					10
Foreign currency translation adjustments	(3,818)					(3,818)
Total comprehensive income (loss), 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	33,062