Stockholders' Equity (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Summary of the changes in stockholders' equity
Balance, December 31, 2010			$ 572,322
Net income	36,213	32,084	107,414	78,029
Dividend payments			(17,395)
Dividends accrued for unvested restricted stock units			(22)
Net change in unrealized gains and losses on marketable securities (net of deferred taxes)	(5,468)	4,230	(11,259)	3,174
Reclassification adjustment for amounts included in net income, net of deferred tax benefit of $195 and $47 for the three months ended September 30, 2011 and 2010, respectively, and $381 and $174 for the nine months ended September 30, 2011 and 2010, respectively	(287)	(80)	(682)	(292)
Foreign currency translation adjustment	(1,487)	1,465	(1,031)	863
Proceeds from stock option exercises			33,631
Tax benefits from stock option exercises			10,457
Stock-based compensation expense			6,455
Purchases of treasury stock			(35,178)
Balance, September 30, 2011	$ 664,712		$ 664,712