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Investments (Schedule of Assets or Liabilities, Recurring Fair Value Measurements) (Details) - USD ($)
Jun. 30, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash & Cash Equivalents $ 260,332,892 $ 217,025,278
Market Value 304,026,286 303,542,081
Investments and Cash 564,359,178 520,567,359
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash & Cash Equivalents 260,332,892 217,025,278
Investments and Cash 411,916,856 369,544,350
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash & Cash Equivalents 0 0
Investments and Cash 152,442,322 151,023,009
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash & Cash Equivalents 0 0
Investments and Cash 0 0
Short-Term Investments | Certificates of Deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 150,299,384 150,299,384
Short-Term Investments | Certificates of Deposit | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 150,299,384 150,299,384
Short-Term Investments | Certificates of Deposit | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Short-Term Investments | Certificates of Deposit | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Short-Term Investments | Government Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 23,953,999 9,176,227
Short-Term Investments | Government Securities | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Short-Term Investments | Government Securities | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 23,953,999 9,176,227
Short-Term Investments | Government Securities | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Short-Term Investments | Corporate Bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 15,027,102 6,967,700
Short-Term Investments | Corporate Bonds | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Short-Term Investments | Corporate Bonds | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 15,027,102 6,967,700
Short-Term Investments | Corporate Bonds | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Short-Term Investments | Other    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 1,284,580 2,219,688
Short-Term Investments | Other | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 1,284,580 2,219,688
Short-Term Investments | Other | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Short-Term Investments | Other | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Long-Term Investments | Government Securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 23,012,586 56,483,720
Long-Term Investments | Government Securities | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Long-Term Investments | Government Securities | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 23,012,586 56,483,720
Long-Term Investments | Government Securities | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Long-Term Investments | Corporate Bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 62,317,048 60,369,930
Long-Term Investments | Corporate Bonds | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Long-Term Investments | Corporate Bonds | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 62,317,048 60,369,930
Long-Term Investments | Corporate Bonds | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Long-Term Investments | Municipal Bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 28,131,587 18,025,432
Long-Term Investments | Municipal Bonds | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 0 0
Long-Term Investments | Municipal Bonds | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value 28,131,587 18,025,432
Long-Term Investments | Municipal Bonds | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Market Value $ 0 $ 0