Comprehensive Income (AOCI Rollforward) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 1,910,424,234	$ 1,722,516,761	$ 1,571,412,445
Comprehensive Income	415,256,610	347,450,807	306,026,008
Ending balance	2,049,518,261	1,910,424,234	1,722,516,761
Accumulated other comprehensive income (loss), end of period	7,193,383	(1,271,305)	(1,130,836)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(2,862,999)	(44,909)	1,403,899
Other comprehensive income (loss) before reclassifications	3,508,029	(2,818,090)	(1,448,808)
Comprehensive Income	3,508,029	(2,818,090)	(1,448,808)
Ending balance	645,030	(2,862,999)	(44,909)
Unrealized gains(losses) on available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	2,788,975	829,907	10,868,322
Other comprehensive income (loss) before reclassifications	4,444,360	2,167,196	(5,513,842)
Amounts reclassified from accumulated other comprehensive income	(606,956)	(208,128)	(4,524,573)
Comprehensive Income	3,837,404	1,959,068	(10,038,415)
Ending balance	6,626,379	2,788,975	829,907
Unrealized gains (losses) on derivatives
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,197,281)	(1,915,834)	(959,206)
Other comprehensive income (loss) before reclassifications	248,042	(672,419)	(1,659,115)
Amounts reclassified from accumulated other comprehensive income	871,213	1,390,972	702,487
Comprehensive Income	1,119,255	718,553	(956,628)
Ending balance	$ (78,026)	$ (1,197,281)	$ (1,915,834)