Investments (Schedule of Amortized Cost, Unrealized Gains And Losses, And Market Value of Investment Securities) (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost	$ 301,641,575	$ 303,467,531
Unrealized Gains	1,169,438	440,807
Unrealized Losses	(69,283)	(366,257)
Market Value	302,741,730	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	21,101,631	9,186,586
Unrealized Gains	9,901	0
Unrealized Losses	(1,664)	(10,359)
Market Value	21,109,868	9,176,227
Short-Term Investments | Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	5,979,056	6,981,305
Unrealized Gains	4,095	0
Unrealized Losses	(463)	(13,605)
Market Value	5,982,688	6,967,700
Short-Term Investments | Other
Debt Securities, Available-for-sale [Line Items]
Cost	2,921,282	2,219,688
Unrealized Gains	0	0
Unrealized Losses	0	0
Market Value	2,921,282	2,219,688
Long-Term Investments | Government Securities
Debt Securities, Available-for-sale [Line Items]
Cost	34,316,447	56,280,552
Unrealized Gains	218,059	205,553
Unrealized Losses	0	(2,385)
Market Value	34,534,506	56,483,720
Long-Term Investments | Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	66,172,642	60,659,498
Unrealized Gains	504,809	50,340
Unrealized Losses	(67,156)	(339,908)
Market Value	66,610,295	60,369,930
Long-Term Investments | Municipal Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	20,851,133	17,840,518
Unrealized Gains	432,574	184,914
Unrealized Losses	0	0
Market Value	$ 21,283,707	$ 18,025,432