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, 2017	$ 342,746	$ 689	$ 9,082	$ 427,091	$ (814,732)	$ (28,822)	$ 749,438
Balance, beginning of period (Cumulative Effect, Period of Adoption, Adjustment) at Dec. 31, 2017	$ 7,562						7,562
Balance, beginning of period, shares at Dec. 31, 2017		33,516
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201409Member
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	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 common stock pursuant to stock-based compensation plans		$ 5		(5)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		553
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		(120)
Stock-based compensation expense	19,358			19,358
Dividends	(28,003)						(28,003)
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
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member
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	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 common stock pursuant to stock-based compensation plans		$ 4		(4)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		513
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		(142)
Stock-based compensation expense	19,919			19,919
Exercise of Comcast 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
Net income	$ 58,711						58,711
Unrealized gain on short-term investments, net of tax	(3)					(3)
Foreign currency translation adjustments	8,368					8,368
Total comprehensive income	67,076
Repurchase of common stock	(38,168)			(11,859)	(26,309)
Repurchase of common stock, shares		(878)
Issuance of common stock pursuant to employee stock purchase plan	2,523			2,523
Issuance of common stock pursuant to employee stock purchase plan, shares		68
Issuance of restricted common stock pursuant to stock-based compensation plans		$ 7		(7)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		672
Cancellation of restricted common stock issued pursuant to stock-based compensation plans	(3)	$ (3)
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(40)
Stock-based compensation expense	25,237			25,237
Dividends	(30,932)						(30,932)
Balance, ending of period at Dec. 31, 2020	$ 422,395	$ 700		$ 470,557	$ (894,126)	$ (31,138)	$ 876,402
Balance, ending of period, shares at Dec. 31, 2020	32,713	32,713