CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - 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, 2016	$ 251,360	$ 672	$ 16,007	$ 391,209	$ (826,002)	$ (45,372)	$ 714,846
Balance, beginning of period, shares at Dec. 31, 2016		32,261
Net income	61,364						61,364
Unrealized gain on short-term investments, net of tax	71					71
Foreign currency translation adjustments	16,479					16,479
Total comprehensive income, net of tax	77,914
Repurchase of common stock	(30,649)	$ (2)		(10,099)	(20,548)
Repurchase of common stock, shares		(749)
Issuance of common stock pursuant to employee stock purchase plan	1,776			1,776
Issuance of common stock pursuant to employee stock purchase plan, shares		50
Issuance of restricted stock pursuant to stock-based compensation plans		$ 6		(6)
Issuance of restricted stock pursuant to stock-based compensation plans, shares		649
Cancellation of restricted stock issued pursuant to stock-based compensation plans, shares		(39)
Stock-based compensation expense	20,996			20,996
Issuance of common stock warrants	9,082		9,082
Exercise of common stock warrants		$ 6	(16,007)	641	15,360
Exercise of common stock warrants, shares		649
Repurchase of convertible notes, net of tax	39,039	$ 7		22,574	16,458
Repurchase of convertible notes, net of tax, shares		695
Dividends	(26,772)						(26,772)
Balance, ending of period at Dec. 31, 2017	342,746	$ 689	9,082	427,091	(814,732)	(28,822)	749,438
Balance, ending of period, shares at Dec. 31, 2017		33,516
Net income	66,130						66,130
Unrealized gain on short-term investments, net of tax	90					90
Foreign currency translation adjustments	(14,203)					(14,203)
Total comprehensive income, net of tax	52,017
Repurchase of common stock	(34,967)			(7,339)	(27,628)
Repurchase of common stock, shares		(862)
Issuance of common stock pursuant to employee stock purchase plan	2,311			2,311
Issuance of common stock pursuant to employee stock purchase plan, shares		71
Issuance of restricted stock pursuant to stock-based compensation plans		$ 5		(5)
Issuance of restricted stock pursuant to stock-based compensation plans, shares		553
Cancellation of restricted stock issued pursuant to stock-based compensation plans		$ (1)		1
Cancellation of restricted stock issued pursuant to stock-based compensation plans, shares		(120)
Stock-based compensation expense	19,358			19,358
Dividends	(28,003)						(28,003)
Adjustments due to adoption of new accounting standards	7,562						7,562
Balance, ending of period at Dec. 31, 2018	$ 361,024	$ 693	9,082	441,417	(842,360)	(42,935)	795,127
Balance, ending of period, shares at Dec. 31, 2018	33,158	33,158
Net income	$ 82,770						82,770
Unrealized gain on short-term investments, net of tax	14					14
Foreign currency translation adjustments	3,418					3,418
Total comprehensive income, net of tax	86,202
Repurchase of common stock	(30,525)			(5,068)	(25,457)
Repurchase of common stock, shares		(693)
Issuance of common stock pursuant to employee stock purchase plan	2,227			2,227
Issuance of common stock pursuant to employee stock purchase plan, shares		55
Issuance of restricted stock pursuant to stock-based compensation plans		$ 4		(4)
Issuance of restricted stock pursuant to stock-based compensation plans, shares		513
Cancellation of restricted stock issued pursuant to stock-based compensation plans		$ (1)		1
Cancellation of restricted stock issued pursuant to stock-based compensation plans, shares		(142)
Stock-based compensation expense	19,919			19,919
Exercise of common stock warrants	(12,911)		$ (9,082)	(3,829)
Dividends	(29,274)						(29,274)
Balance, ending of period at Dec. 31, 2019	$ 396,662	$ 696		$ 454,663	$ (867,817)	$ (39,503)	$ 848,623
Balance, ending of period, shares at Dec. 31, 2019	32,891	32,891