Comprehensive Income (AOCI Rollforward) (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	[1]			$ 2,049,518,261
Comprehensive Income		$ 109,692,931	$ 93,054,594	328,935,897	$ 283,599,743
Balance at end of period		1,843,104,255		1,843,104,255
Accumulated other comprehensive income (loss), end of period
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at end of period		(2,121,990)	6,005,415	(2,121,990)	6,005,415
Foreign currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(194,395)	(1,179,199)	645,030	(2,862,999)
Other Comprehensive income (loss) before reclassifications		(1,174,213)	1,182,152	(2,013,638)	2,865,952
Comprehensive Income		(1,174,213)	1,182,152	(2,013,638)	2,865,952
Balance at end of period		(1,368,608)	2,953	(1,368,608)	2,953
Unrealized gains (losses) on available-for-sale debt securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(271,939)	4,818,677	(16,349)	2,788,975
Other Comprehensive income (loss) before reclassifications		(276,010)	1,371,712	33,941	4,111,338
Amounts reclassified from accumulated other comprehensive income		(205,433)	114,905	(770,974)	(595,019)
Comprehensive Income		(481,443)	1,486,617	(737,033)	3,516,319
Balance at end of period		(753,382)	6,305,294	(753,382)	6,305,294
Unrealized gains (losses) on derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(12,770)	(458,322)	(78,026)	(1,197,281)
Other Comprehensive income (loss) before reclassifications		114,889	424	175,308	124,124
Amounts reclassified from accumulated other comprehensive income		(102,119)	155,066	(97,282)	770,325
Comprehensive Income		12,770	155,490	78,026	894,449
Balance at end of period		$ 0	$ (302,832)	$ 0	$ (302,832)

[1] The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements.