Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Treasury Stock	Retained Earnings	Total
Balance at Dec. 31, 2011	$ 615	$ 992,718	$ (10,885)	$ (282,998)	$ 249,038	$ 948,488
Balance (in shares) at Dec. 31, 2011	61,506,063
Increase (Decrease) in Stockholders' Equity
Net income					304,442	304,442
Foreign currency translation adjustments			691			691
Unrealized gain (loss) on available-for-sale securities			67			67
Defined benefit pension plan			(4,830)			(4,830)
Repurchase of shares				(188,000)		(188,000)
Exercise of stock options	6	16,799				16,805
Exercise of stock options (in shares)	575,944					575,944
Tax benefit from exercises of non-qualified stock options		3,054				3,054
Share-based compensation		3,264				3,264
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 gain (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 gain (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