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, 2014	$ 358,633	$ 667	$ 6,694	$ 486,414	$ (757,478)	$ (13,380)	$ 635,716
Balance, beginning of period, shares at Dec. 31, 2014		33,945
Net income	62,567						62,567
Unrealized gain (loss) on short-term investments, net of tax	(103)					(103)
Foreign currency translation adjustments	(12,902)					(12,902)
Total comprehensive income, net of tax	49,562
Repurchase of common stock	(65,027)	$ (3)		(8,065)	(56,959)
Repurchase of common stock, shares		(2,103)
Issuance of common stock pursuant to employee stock purchase plan	1,547	$ 1		1,546
Issuance of common stock pursuant to employee stock purchase plan, shares		59
Stock-based compensation income tax benefits	2,185			2,185
Issuance of restricted common stock pursuant to stock-based compensation plans		$ 8		(8)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		728
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		(74)
Stock-based compensation expense	21,181			21,181
Issuance of common stock warrants, granted to Comcast	616		616
Declaration of cash dividends	(22,852)						(22,852)
Balance, ending of period at Dec. 31, 2015	345,845	$ 672	7,310	503,254	(814,437)	(26,385)	675,431
Balance, ending of period, shares at Dec. 31, 2015		32,555
Net income	62,882						62,882
Unrealized gain (loss) on short-term investments, net of tax	(62)					(62)
Foreign currency translation adjustments	(18,925)					(18,925)
Total comprehensive income, net of tax	43,895
Repurchase of common stock	(25,196)	$ (3)		(13,628)	(11,565)
Repurchase of common stock, shares		(662)
Issuance of common stock pursuant to employee stock purchase plan	1,547			1,547
Issuance of common stock pursuant to employee stock purchase plan, shares		44
Stock-based compensation income tax benefits	4,729			4,729
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		572
Cancellation of restricted common stock issued pursuant to stock-based compensation plans		$ (2)		2
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(248)
Stock-based compensation expense	22,745			22,745
Issuance of common stock warrants, granted to Comcast	8,697		8,697
Issuance of convertible notes	9,816			9,816
Repurchase of convertible notes, net of tax	(97,410)			(97,410)
Debt conversion obligation reclassification	(39,841)			(39,841)
Declaration of cash dividends	(23,467)						(23,467)
Balance, ending of period at Dec. 31, 2016	$ 251,360	$ 672	16,007	391,209	(826,002)	(45,372)	714,846
Balance, ending of period, shares at Dec. 31, 2016	32,261	32,261
Net income	$ 61,364						61,364
Unrealized gain (loss) 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 common stock pursuant to stock-based compensation plans		$ 6		(6)
Issuance of restricted common stock pursuant to stock-based compensation plans, shares		649
Cancellation of restricted common stock issued pursuant to stock-based compensation plans, shares		(39)
Stock-based compensation expense	20,996			20,996
Issuance of common stock warrants, granted to Comcast	9,082		9,082
Exercise of common stock warrants, granted to Comcast		$ 6	(16,007)	641	15,360
Exercise of common stock warrants, granted to Comcast, 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
Declaration of cash 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	33,516