Unaudited Condensed Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 72,336,883	$ 55,520,961	$ 217,629,700	$ 153,048,925
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	(269,392)	273,097	(602,131)	(99,537)
Unrealized gains (losses) on available-for sales securities, net	(4,785,501)	1,598,977	(11,131,487)	7,119,498
Other comprehensive income (loss), before tax	(5,054,893)	1,872,074	(11,733,618)	7,019,961
Provision (Benefit) for income taxes related to components of other comprehensive income	(1,674,925)	559,642	(3,896,021)	2,491,824
Other comprehensive income (loss), net of tax	(3,379,968)	1,312,432	(7,837,597)	4,528,137
Comprehensive Income	$ 68,956,915	$ 56,833,393	$ 209,792,103	$ 157,577,062