Consolidated Statement of Comprehensive Income - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 437,883,097	$ 406,791,922	$ 347,591,276
Other comprehensive income (loss) before tax:
Foreign currency translation adjustments	(2,319,917)	3,508,029	(2,818,090)
Unrealized gains on derivatives	98,767	1,721,933	1,105,468
Unrealized gains on available-for-sale securities, net	115,059	5,903,699	3,013,951
Other comprehensive income (loss), before tax	(2,106,091)	11,133,661	1,301,328
Expense for income taxes related to components of other comprehensive income (loss)	44,903	2,668,973	1,441,798
Other comprehensive income (loss), net of tax	(2,150,994)	8,464,688	(140,469)
Comprehensive income	$ 435,732,103	$ 415,256,610	$ 347,450,807