Investments (Schedule of Amortized Cost, Unrealized Gains and Losses, and Market Value of Investment Securities) (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost	$ 298,869,584	$ 303,467,531
Unrealized Gains	2,730,098	440,807
Unrealized Losses	(17,927)	(366,257)
Market Value	301,581,755	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	38,067,382	9,186,586
Unrealized Gains	97,271	0
Unrealized Losses	(1,300)	(10,359)
Market Value	38,163,353	9,176,227
Short-Term Investments | Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	15,955,794	6,981,305
Unrealized Gains	94,766	0
Unrealized Losses	0	(13,605)
Market Value	16,050,560	6,967,700
Short-Term Investments | Other
Debt Securities, Available-for-sale [Line Items]
Cost	971,726	2,219,688
Unrealized Gains	0	0
Unrealized Losses	0	0
Market Value	971,726	2,219,688
Short-Term Investments | Municipal Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	1,673,133
Unrealized Gains	13,163
Unrealized Losses	(15,032)
Market Value	1,671,264
Long-Term Investments | Government Securities
Debt Securities, Available-for-sale [Line Items]
Cost	11,539,947	56,280,552
Unrealized Gains	97,563	205,553
Unrealized Losses	0	(2,385)
Market Value	11,637,510	56,483,720
Long-Term Investments | Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	41,191,483	60,659,498
Unrealized Gains	1,130,771	50,340
Unrealized Losses	(1,595)	(339,908)
Market Value	42,320,659	60,369,930
Long-Term Investments | Municipal Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	39,170,735	17,840,518
Unrealized Gains	1,296,564	184,914
Unrealized Losses	0	0
Market Value	$ 40,467,299	$ 18,025,432