CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Gain (Loss)	1.125% Convertible Senior Notes due 2018	1.125% Convertible Senior Notes due 2018 Additional Paid-in Capital	Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]	Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member] Additional Paid-in Capital
Net income (loss)	$ 211,388	$ 0	$ 0	$ 211,388	$ 0
Foreign currency translation adjustment	9				9
Balance at Dec. 31, 2014	391,622	$ 115	1,153,435	(761,526)	(402)
Balance (in shares) at Dec. 31, 2014		115,162
Increase (Decrease) in Stockholders' Equity
Unrealized gain (loss) on marketable securities, net of tax	766	$ 0	0	0	766
Issuance of common stock upon exercise of options, equity stock and employee stock purchase plan	$ 13,077	$ 2	13,075	0	0
Issuance of common stock upon exercise of options, equity stock and stock units, and employee stock purchase plan (in shares)		1,938
Stock Repurchased and Retired During Period, Shares	(7,812)	(7,812)
Stock Repurchased and Retired During Period, Value	$ (80,000)	$ (8)	(45,926)	(54,179)	0
Repurchase and retirement of common stock under repurchase plan, including prepayment under accelerated share repurchase program	(100,113)
Stock-based compensation	15,096	0	15,096	0	0
Tax shortfall from stock option forfeitures	(5,312)	0	(5,312)	0	0
Balance at Dec. 31, 2015	526,533	$ 109	1,130,368	(604,317)	373
Balance (in shares) at Dec. 31, 2015		109,288
Net income (loss)	6,820	$ 0	0	6,820	0
Foreign currency translation adjustment	(13,485)				(13,485)
Increase (Decrease) in Stockholders' Equity
Unrealized gain (loss) on marketable securities, net of tax	(396)	0	0	0	(396)
Issuance of common stock upon exercise of options, equity stock and employee stock purchase plan	12,297	$ 3	12,294	0	0
Issuance of common stock upon exercise of options, equity stock and stock units, and employee stock purchase plan (in shares)		2,502
Stock Repurchased and Retired During Period, Shares		(736)
Stock Repurchased and Retired During Period, Value		$ (1)	17,555	(17,554)	0
Repurchase and retirement of common stock under repurchase plan, including prepayment under accelerated share repurchase program	0
Stock-based compensation	21,013	0	21,013	0	0
Balance at Dec. 31, 2016	552,782	$ 111	1,181,230	(615,051)	(13,508)
Balance (in shares) at Dec. 31, 2016		111,054
Net income (loss)	(22,862)	$ 0	0	(22,862)	0
Foreign currency translation adjustment	7,798				7,798
Increase (Decrease) in Stockholders' Equity
Unrealized gain (loss) on marketable securities, net of tax	613	0	0	0	613
Issuance of common stock upon exercise of options, equity stock and employee stock purchase plan	10,733	$ 3	10,730	0	0
Issuance of common stock upon exercise of options, equity stock and stock units, and employee stock purchase plan (in shares)		2,727
Stock Repurchased and Retired During Period, Shares		(4,017)
Stock Repurchased and Retired During Period, Value	(36,557)	$ (4)	(13,477)	(36,557)	0
Repurchase and retirement of common stock under repurchase plan, including prepayment under accelerated share repurchase program	(50,038)
Stock-based compensation	27,403	0	27,403	0	0
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt						$ (16,651)	$ (16,651)	$ 33,913	$ 33,913
Adjustments to Additional Paid in Capital, Purchase of convertible note hedges	(33,523)								$ (33,523)
Adjustments to Additional Paid in Capital, Warrant Issued	23,173		23,173
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income	38,243			38,243
Balance at Dec. 31, 2017	$ 571,584	$ 110	$ 1,212,798	$ (636,227)	$ (5,097)
Balance (in shares) at Dec. 31, 2017		109,764