Condensed Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement of Other Comprehensive Income [Abstract]
Net income	$ 55,520,961	$ 41,877,083	$ 153,048,925	$ 128,988,206
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	273,097	275,648	(99,537)	(164,559)
Unrealized gains on available-for sales securities, net	1,598,977	4,198,182	7,119,498	8,829,548
Other comprehensive income, before tax	1,872,074	4,473,830	7,019,961	8,664,989
Provision for income taxes related to components of other comprehensive income	559,642	1,469,364	2,491,824	3,090,342
Other comprehensive income, net of tax	1,312,432	3,004,466	4,528,137	5,574,647
Comprehensive Income	$ 56,833,393	$ 44,881,549	$ 157,577,062	$ 134,562,853