Consolidated Statement of Comprehensive Income - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 406,791,922	$ 347,591,276	$ 318,469,859
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	3,508,029	(2,818,090)	(1,448,808)
Unrealized gains (losses) on derivatives	1,721,933	1,105,468	(1,471,736)
Unrealized gains (losses) on available-for-sale securities, net	5,903,699	3,013,951	(15,443,716)
Other comprehensive income (loss), before tax	11,133,661	1,301,328	(18,364,260)
Expense (Benefit) for income taxes related to components of other comprehensive income (loss)	2,668,973	1,441,798	(5,920,409)
Other comprehensive income (loss), net of tax	8,464,688	(140,469)	(12,443,851)
Comprehensive Income	$ 415,256,610	$ 347,450,807	$ 306,026,008