Consolidated Statement of Comprehensive Income - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net Income	$ 318,469,859	$ 288,604,579	$ 222,929,949
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	(1,448,808)	(1,104,023)	48,989
Unrealized gains (losses) on derivatives	(1,471,736)	(1,475,702)	0
Unrealized gains (losses) on available-for-sale securities, net	(15,443,716)	(13,229,414)	6,701,020
Other comprehensive income (loss), before tax	(18,364,260)	(15,809,139)	6,750,009
Provision for income taxes related to components of other comprehensive income	(5,920,409)	(5,146,791)	2,345,357
Other comprehensive Income (loss), net of tax	(12,443,851)	(10,662,348)	4,404,652
Comprehensive Income	$ 306,026,008	$ 277,942,231	$ 227,334,601