Comprehensive Income (AOCI Rollforward) (Details) - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	[1]	$ 2,049,518,261
Comprehensive Income		112,911,077	$ 99,573,280
Balance at end of period		1,969,977,496
Accumulated other comprehensive income (loss), end of period
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at end of period		2,212,989	745,234
Foreign currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		645,030	(2,862,999)
Other Comprehensive income (loss) before reclassifications		1,626,178	188,590
Comprehensive Income		1,626,178	188,590
Balance at end of period		2,271,208	(2,674,409)
Unrealized gains (losses) on available-for-sale debt securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(16,349)	2,788,975
Other Comprehensive income (loss) before reclassifications		548,506	1,575,383
Amounts reclassified from accumulated other comprehensive income		(565,317)	(164,718)
Comprehensive Income		(16,811)	1,410,665
Balance at end of period		(33,160)	4,199,640
Unrealized gains (losses) on derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(78,026)	(1,197,281)
Other Comprehensive income (loss) before reclassifications		43,173	147,318
Amounts reclassified from accumulated other comprehensive income		9,794	269,966
Comprehensive Income		52,967	417,284
Balance at end of period		$ (25,059)	$ (779,997)

[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.