Unaudited Condensed Consolidated Statements of Comprehensive Income - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 78,332,425	$ 72,336,883	$ 230,074,303	$ 217,629,700
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	(396,686)	(269,392)	(1,038,892)	(602,131)
Unrealized losses on derivatives	(1,275,655)	0	(3,143,576)	0
Unrealized losses on available-for sales securities, net	(11,808,718)	(4,785,501)	(15,372,962)	(11,131,487)
Other comprehensive loss, before tax	(13,481,059)	(5,054,893)	(19,555,430)	(11,733,618)
Benefit for income taxes related to components of other comprehensive loss	(4,579,531)	(1,674,925)	(6,480,789)	(3,896,021)
Other comprehensive loss, net of tax	(8,901,528)	(3,379,968)	(13,074,641)	(7,837,597)
Comprehensive Income	$ 69,430,897	$ 68,956,915	$ 216,999,662	$ 209,792,103