Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Treasury Stock	Retained Earnings	Total
Balance at Dec. 31, 2012	$ 621	$ 1,015,835	$ (14,957)	$ (470,998)	$ 553,480	$ 1,083,981
Balance (in shares) at Dec. 31, 2012	62,082,007
Increase (Decrease) in Stockholders' Equity
Net income					174,560	174,560
Foreign currency translation adjustments			(1,193)			(1,193)
Unrealized (loss) on available-for-sale securities			(128)			(128)
Defined benefit pension plan			3,095			3,095
Shares issued under employee stock purchase plan	$ 1	2,734				2,735
Shares issued under employee stock purchase plan (in shares)	55,070
Equity component - 2016 convertible notes (Note 10)		(34,155)				(34,155)
Repurchase of shares				(42,439)		(42,439)
Exercise of stock options	$ 8	26,611				$ 26,619
Exercise of stock options (in shares)	876,115					876,115
Tax benefit from exercises of non-qualified stock options		9,299				$ 9,299
Share-based compensation		36,900				36,900
Balance at Dec. 31, 2013	$ 630	1,057,224	(13,183)	(513,437)	728,040	1,259,274
Balance (in shares) at Dec. 31, 2013	63,013,192
Increase (Decrease) in Stockholders' Equity
Net income					340,074	340,074
Foreign currency translation adjustments			(4,789)			(4,789)
Unrealized (loss) on available-for-sale securities			(250)			(250)
Defined benefit pension plan			1,488			1,488
Shares issued under employee stock purchase plan	$ 1	3,329				3,330
Shares issued under employee stock purchase plan (in shares)	45,657
Conversion of 2016 convertible notes (Note 10)	$ 15	192,966		(189,311)		3,670
Conversion of 2016 convertible notes (Note 10) (in shares)	1,467,343
Equity component - 2016 convertible notes (Note 10)		11,056				11,056
Repurchase of shares				(483,077)		(483,077)
Exercise of stock options	$ 14	50,154				$ 50,168
Exercise of stock options (in shares)	1,462,369					1,462,369
Tax benefit from exercises of non-qualified stock options		30,845				$ 30,845
Share-based compensation		30,567				30,567
Balance at Dec. 31, 2014	$ 660	1,376,141	(16,734)	(1,185,825)	1,068,114	1,242,356
Balance (in shares) at Dec. 31, 2014	65,988,561
Increase (Decrease) in Stockholders' Equity
Net income					651,639	651,639
Foreign currency translation adjustments			(5,296)			(5,296)
Unrealized (loss) on available-for-sale securities			(74)			(74)
Defined benefit pension plan			1,703			1,703
Shares issued under employee stock purchase plan		3,954				3,954
Shares issued under employee stock purchase plan (in shares)	36,198
Conversion of 2016 convertible notes (Note 10)	$ 20	324,701		(321,801)		2,920
Conversion of 2016 convertible notes (Note 10) (in shares)	1,977,577
Equity component - 2016 convertible notes (Note 10)		3,020				3,020
Repurchase of shares				(394,484)		(394,484)
Exercise of stock options	$ 10	39,301				$ 39,311
Exercise of stock options (in shares)	985,583					985,583
Tax benefit from exercises of non-qualified stock options		37,426				$ 37,426
Share-based compensation		6,077				6,077
Balance at Dec. 31, 2015	$ 690	$ 1,790,620	$ (20,401)	$ (1,902,110)	$ 1,719,753	$ 1,588,552
Balance (in shares) at Dec. 31, 2015	68,987,919