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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, Fair Value Measurements (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Fair Value Measurements [Abstract]    
Held-to-maturity securities, unrecognized gain $ 0 $ 0
Held-to-maturity securities, unrecognized (loss) 0 0
Amortized net discount (premium) included in interest income 50,000 (372,000)
Debt Securities, Held-to-maturity [Abstract]    
Held-to-maturity securities, current 66,899,660 67,707,143
Held-to-maturity securities, noncurrent 3,635,115 1,099,834
Recurring [Member] | Institutional Money Market [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 10,370,601 6,078,025
Recurring [Member] | Municipal Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 330,000  
Investments 11,560,848 1,295,350
Recurring [Member] | Corporate Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 53,222,465 62,420,996
Recurring [Member] | Certificates of Deposit [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 5,751,462 5,090,631
Recurring [Member] | Level 1 [Member] | Institutional Money Market [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 10,370,601 6,078,025
Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 330,000  
Investments 0 0
Recurring [Member] | Level 1 [Member] | Corporate Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 0 0
Recurring [Member] | Level 1 [Member] | Certificates of Deposit [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 5,751,462 5,090,631
Recurring [Member] | Level 2 [Member] | Institutional Money Market [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 0 0
Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 0  
Investments 11,560,848 1,295,350
Recurring [Member] | Level 2 [Member] | Corporate Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 53,222,465 62,420,996
Recurring [Member] | Level 2 [Member] | Certificates of Deposit [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 0 0
Recurring [Member] | Level 3 [Member] | Institutional Money Market [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 0 0
Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Cash equivalents 0  
Investments 0 0
Recurring [Member] | Level 3 [Member] | Corporate Bonds [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 0 0
Recurring [Member] | Level 3 [Member] | Certificates of Deposit [Member]    
Assets, Fair Value Disclosure [Abstract]    
Investments 0 0
Municipal Bonds [Member]    
Debt Securities, Held-to-maturity [Abstract]    
Held-to-maturity securities, current 11,560,848 1,295,350
Held-to-maturity securities, noncurrent 0 0
Corporate Bonds [Member]    
Debt Securities, Held-to-maturity [Abstract]    
Held-to-maturity securities, current 49,787,288 61,321,162
Held-to-maturity securities, noncurrent 3,435,177 1,099,834
Certificates of Deposit [Member]    
Debt Securities, Held-to-maturity [Abstract]    
Held-to-maturity securities, current 5,551,524 5,090,631
Held-to-maturity securities, noncurrent $ 199,938 $ 0