Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data	Total	Common stock [Member]	Additional Paid-in capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Beginning Balances at Dec. 31, 2008	$ 872,659	$ 539	$ 466,926	$ (4,022)	$ 441,353	$ (32,137)
Beginning Balances, Shares at Dec. 31, 2008		52,673,363				1,207,510
Common stock issued upon exercise of stock options, vesting of restricted stock grants, and purchases under the employee stock purchase plan	11,041	7	11,034
Common stock issued upon exercise of stock options, vesting of restricted stock grants, and purchases under the employee stock purchase plan, Shares		714,161
Compensation expense related to share-based payments	15,936		15,936
Tax benefit related to share-based payments	842		842
Employee stock relinquished for payment of taxes	(591)					(591)
Employee stock relinquished for payment of taxes, Shares		(24,161)				24,161
Employee stock relinquished for stock option exercises	(344)					(344)
Employee stock relinquished for stock option exercises, Shares		(11,322)				11,322
Common stock repurchases	(69,751)					(69,751)
Common stock repurchases, Shares		(2,713,567)				2,713,567
Unrealized gain on held-to-maturity investment portfolio at time of transfer to available-for-sale, net of tax	3,030			3,030
Unrealized gain (loss) on available-for-sale securities, net of tax	2,346			2,346
Other	(3)		(3)
Net income	149,279				149,279
Ending Balances at Dec. 31, 2009	984,444	546	494,735	1,354	590,632	(102,823)
Ending Balances, Shares at Dec. 31, 2009		50,638,474				3,956,560
Common stock issued upon exercise of stock options, vesting of restricted stock grants, and purchases under the employee stock purchase plan	26,466	8	26,458
Common stock issued upon exercise of stock options, vesting of restricted stock grants, and purchases under the employee stock purchase plan, Shares		1,331,429
Compensation expense related to share-based payments	19,635		19,635
Tax benefit related to share-based payments	3,097		3,097
Employee stock relinquished for payment of taxes	(1,795)					(1,795)
Employee stock relinquished for payment of taxes, Shares		(54,005)				54,005
Common stock repurchases	(138,540)					(138,540)
Common stock repurchases, Shares		(3,748,669)				3,748,669
Unrealized gain (loss) on available-for-sale securities, net of tax	(727)			(727)
Other	(314)		(314)
Net income	273,371				273,371
Ending Balances at Dec. 31, 2010	1,165,637	554	543,611	627	864,003	(243,158)
Ending Balances, Shares at Dec. 31, 2010		48,167,229				7,759,234
Common stock issued upon exercise of stock options, vesting of restricted stock grants, and purchases under the employee stock purchase plan	52,554	19	52,535
Common stock issued upon exercise of stock options, vesting of restricted stock grants, and purchases under the employee stock purchase plan, Shares		2,153,645
Compensation expense related to share-based payments	22,868		22,868
Tax benefit related to share-based payments	18,593		18,593
Employee stock relinquished for payment of taxes	(6,349)					(6,349)
Employee stock relinquished for payment of taxes, Shares		(102,932)				102,932
Common stock repurchases	(175,718)					(175,718)
Common stock repurchases, Shares		(3,339,468)				3,339,468
Unrealized gain (loss) on available-for-sale securities, net of tax	11,315			11,315
Other	(2)		(2)
Net income	195,621				195,621
Ending Balances at Dec. 31, 2011	$ 1,284,519	$ 573	$ 637,605	$ 11,942	$ 1,059,624	$ (425,225)
Ending Balances, Shares at Dec. 31, 2011		46,878,474				11,201,634