Condensed Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Net income	$ 52,097,397	$ 40,766,539	$ 97,527,964	$ 87,111,123
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	229,115	(784,992)	(372,634)	(440,207)
Unrealized gains (losses) on available-for sales securities, net	(3,674,340)	(8,882,763)	5,520,521	4,631,367
Other comprehensive income (loss), before tax	(3,445,225)	(9,667,755)	5,147,887	4,191,160
Provision for income taxes related to components of other comprehensive income	(1,286,019)	(3,108,967)	1,932,182	1,620,978
Other comprehensive income (loss), net of tax	(2,159,206)	(6,558,788)	3,215,705	2,570,182
Comprehensive Income	$ 49,938,191	$ 34,207,751	$ 100,743,669	$ 89,681,305