Unaudited Condensed Consolidated Statements of Comprehensive Income - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 74,558,437	$ 76,726,746	$ 151,741,878	$ 145,292,817
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	300,766	(241,489)	(642,207)	(332,739)
Unrealized gains (losses) on derivatives	(36,229)	0	(1,867,920)	0
Unrealized gains (losses) on available-for sales securities, net	(2,986,559)	(2,653,194)	(3,564,244)	(6,345,985)
Other comprehensive income (loss), before tax	(2,722,022)	(2,894,683)	(6,074,371)	(6,678,724)
Provision (Benefit) for income taxes related to components of other comprehensive income	(1,057,977)	(928,618)	(1,901,258)	(2,221,095)
Other comprehensive income (loss), net of tax	(1,664,045)	(1,966,065)	(4,173,113)	(4,457,629)
Comprehensive Income	$ 72,894,392	$ 74,760,681	$ 147,568,765	$ 140,835,188