Consolidated Statement of Comprehensive Income (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Other Comprehensive Income [Abstract]
Net Income	$ 168,586,840	$ 164,668,228	$ 137,733,969
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	328,976	1,563,054	(2,176,851)
Unrealized gains (losses) on available-for sales securities, net	7,230,048	(17,442,698)	9,468,310
Other comprehensive income (loss), before tax	7,559,024	(15,879,644)	7,291,459
Provision for income taxes related to components of other comprehensive income	2,530,516	(6,104,944)	3,313,898
Other comprehensive Income (loss), net of tax	5,028,508	(9,774,700)	3,977,561
Comprehensive Income	$ 173,615,348	$ 154,893,528	$ 141,711,530