Investments (Schedule of Amortized Cost, Unrealized Gains and Losses, and Market Value of Investment Securities) (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost	$ 301,675,489	$ 303,467,531
Unrealized Gains	2,391,676	440,807
Unrealized Losses	(40,879)	(366,257)
Market Value	304,026,286	303,542,081
Short-Term Investments | Certificates of Deposit
Debt Securities, Available-for-sale [Line Items]
Cost	150,299,384	150,299,384
Unrealized Gains	0	0
Unrealized Losses	0	0
Market Value	150,299,384	150,299,384
Short-Term Investments | Government Securities
Debt Securities, Available-for-sale [Line Items]
Cost	23,895,390	9,186,586
Unrealized Gains	59,368	0
Unrealized Losses	(759)	(10,359)
Market Value	23,953,999	9,176,227
Short-Term Investments | Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	14,968,562	6,981,305
Unrealized Gains	58,540	0
Unrealized Losses	0	(13,605)
Market Value	15,027,102	6,967,700
Short-Term Investments | Other
Debt Securities, Available-for-sale [Line Items]
Cost	1,284,580	2,219,688
Unrealized Gains	0	0
Unrealized Losses	0	0
Market Value	1,284,580	2,219,688
Long-Term Investments | Government Securities
Debt Securities, Available-for-sale [Line Items]
Cost	22,785,296	56,280,552
Unrealized Gains	227,290	205,553
Unrealized Losses	0	(2,385)
Market Value	23,012,586	56,483,720
Long-Term Investments | Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	61,354,213	60,659,498
Unrealized Gains	996,062	50,340
Unrealized Losses	(33,227)	(339,908)
Market Value	62,317,048	60,369,930
Long-Term Investments | Municipal Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	27,088,064	17,840,518
Unrealized Gains	1,050,416	184,914
Unrealized Losses	(6,893)	0
Market Value	$ 28,131,587	$ 18,025,432