Consolidated Statements Of Stockholders' Equity And Comprehensive Income (USD $) In Thousands, except Share data	Common Stock [Member]	Additional Paid-In Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]	Retained Earnings [Member]	Total
Beginning Balance at Dec. 31, 2008	$ 62	$ 338,577	$ (100,020)	$ 84	$ 108,452	$ 347,155
Beginning Balance (in shares) at Dec. 31, 2008	58,862,478
Net income					115,860	115,860
Unrealized gains (losses) on available-for-sale securities, net of taxes				189		189
Comprehensive income, net of taxes						116,049
Issuance of common stock upon exercise of options (in shares)	1,724,110					1,724,110
Issuance of common stock upon exercise of options	1	29,508				29,509
Issuance of common stock under employee stock purchase plan (in shares)	224,799
Issuance of common stock under employee stock purchase plan		5,765				5,765
Repurchases of common stock (in shares)	(7,371,314)					(7,371,000)
Repurchases of common stock	(10)	(398,850)	100,020		(25,495)	(324,335)
Stock-based compensation expense		12,618				12,618
Excess stock option income tax benefits		12,382				12,382
Ending Balance at Dec. 31, 2009	53			273	198,817	199,143
Ending Balance (in shares) at Dec. 31, 2009	53,440,073
Net income					160,853	160,853
Unrealized gains (losses) on available-for-sale securities, net of taxes				477		477
Comprehensive income, net of taxes						161,330
Issuance of common stock upon exercise of options (in shares)	1,902,073					1,902,073
Issuance of common stock upon exercise of options	2	47,080				47,082
Issuance of common stock under employee stock purchase plan (in shares)	46,112
Issuance of common stock under employee stock purchase plan		2,694				2,694
Repurchases of common stock (in shares)	(2,606,309)					(2,606,000)
Repurchases of common stock	(2)	(88,326)			(121,931)	(210,259)
Stock-based compensation expense		27,996				27,996
Excess stock option income tax benefits		62,178				62,178
Ending Balance at Dec. 31, 2010	53	51,622		750	237,739	290,164
Ending Balance (in shares) at Dec. 31, 2010	52,781,949
Net income					226,126	226,126
Unrealized gains (losses) on available-for-sale securities, net of taxes				(68)		(68)
Cumulative translation adjustment				24		24
Comprehensive income, net of taxes						226,082
Issuance of common stock upon exercise of options (in shares)	659,370					659,370
Issuance of common stock upon exercise of options		19,614				19,614
Issuance of common stock, net of costs	3	199,483				199,486
Issuance of common stock, net of costs (in shares)	2,857,143
Repurchases of common stock (in shares)	(899,847)					(900,000)
Repurchases of common stock	(1)	(158,730)			(40,935)	(199,666)
Stock-based compensation expense		61,582				61,582
Excess stock option income tax benefits		45,548				45,548
Ending Balance at Dec. 31, 2011	$ 55	$ 219,119		$ 706	$ 422,930	$ 642,810
Ending Balance (in shares) at Dec. 31, 2011	55,398,615