Investments (Schedule of Amortized Cost, Unrealized Gains And Losses, And Market Value of Investment Securities) (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 309,398,439	$ 196,926,043
Unrealized Gains	48,196	10,562,267
Unrealized Losses	(320,134)	(367,837)
Market Value	309,126,501	207,120,473
Short-Term Investments | Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Cost	160,003,990	130,000,000
Unrealized Gains	13,675	0
Unrealized Losses	(25)	0
Market Value	160,017,640	130,000,000
Short-Term Investments | Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Cost	3,011,806	12,952,229
Unrealized Gains	0	0
Unrealized Losses	(11,085)	(7,230)
Market Value	3,000,721	12,944,999
Short-Term Investments | Government Securities
Schedule of Available-for-sale Securities [Line Items]
Cost	6,693,257	9,024,777
Unrealized Gains	280	0
Unrealized Losses	(1,712)	(13,647)
Market Value	6,691,825	9,011,130
Short-Term Investments | Mutual Funds
Schedule of Available-for-sale Securities [Line Items]
Cost		392,482
Unrealized Gains		1,575
Unrealized Losses		(476)
Market Value		393,581
Short-Term Investments | Other
Schedule of Available-for-sale Securities [Line Items]
Cost	1,310,620	188,344
Unrealized Gains	0	0
Unrealized Losses	0	0
Market Value	1,310,620	188,344
Long-Term Investments | Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Cost	64,584,256	3,022,994
Unrealized Gains	5,591	0
Unrealized Losses	(193,654)	(4,274)
Market Value	64,396,193	3,018,720
Long-Term Investments | Government Securities
Schedule of Available-for-sale Securities [Line Items]
Cost	55,912,057
Unrealized Gains	22,868
Unrealized Losses	(37,044)
Market Value	55,897,881
Long-Term Investments | Mutual Funds
Schedule of Available-for-sale Securities [Line Items]
Cost		29,306,540
Unrealized Gains		5,440,344
Unrealized Losses		(65,547)
Market Value		34,681,337
Long-Term Investments | Common Stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	40,000	10,897,219
Unrealized Gains	0	5,079,815
Unrealized Losses	0	(273,663)
Market Value	40,000	15,703,371
Long-Term Investments | Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Cost	17,842,453
Unrealized Gains	5,782
Unrealized Losses	(76,614)
Market Value	$ 17,771,621
Long-Term Investments | Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Cost		1,141,458
Unrealized Gains		40,533
Unrealized Losses		(3,000)
Market Value		$ 1,178,991