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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, Fair Value Measurements (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Fair Value Measurements          
Held-to-maturity securities, unrecognized gain $ 0   $ 0   $ 0
Held-to-maturity securities, unrecognized (loss) 0   0   0
Amortized net premium / (net bond discount) included in interest income 266,000 $ 32,000 400,000 $ 82,000  
Remaining unamortized net premium / (net discount) 341,000   341,000   285,000
Held-to-maturity Securities          
Held-to-maturity securities, current 82,180,784   82,180,784   84,239,918
Held-to-maturity securities, noncurrent 20,791,278   20,791,278   16,569,024
Recurring | Institutional money market          
Fair Value Measurements          
Cash equivalents 8,058,112   8,058,112   950,658
Recurring | Certificates of deposit          
Fair Value Measurements          
Investments 6,056,611   6,056,611   7,287,409
Recurring | Municipal bonds          
Fair Value Measurements          
Cash equivalents 2,248,333   2,248,333    
Investments 12,761,402   12,761,402   11,341,249
Recurring | Government agency bonds          
Fair Value Measurements          
Investments 6,915,943   6,915,943   18,182,542
Recurring | US Treasury bonds          
Fair Value Measurements          
Investments 6,016,914   6,016,914    
Recurring | Corporate bonds          
Fair Value Measurements          
Investments 71,221,192   71,221,192   63,997,742
Recurring | Level 1 | Institutional money market          
Fair Value Measurements          
Cash equivalents 8,058,112   8,058,112   950,658
Recurring | Level 1 | Certificates of deposit          
Fair Value Measurements          
Investments 6,056,611   6,056,611   7,287,409
Recurring | Level 1 | Municipal bonds          
Fair Value Measurements          
Investments 0   0   0
Recurring | Level 1 | Government agency bonds          
Fair Value Measurements          
Investments 0   0   0
Recurring | Level 1 | US Treasury bonds          
Fair Value Measurements          
Investments 0   0    
Recurring | Level 1 | Corporate bonds          
Fair Value Measurements          
Investments 0   0   0
Recurring | Level 2 | Institutional money market          
Fair Value Measurements          
Cash equivalents 0   0   0
Recurring | Level 2 | Certificates of deposit          
Fair Value Measurements          
Investments 0   0   0
Recurring | Level 2 | Municipal bonds          
Fair Value Measurements          
Cash equivalents 2,248,333   2,248,333    
Investments 12,761,402   12,761,402   11,341,249
Recurring | Level 2 | Government agency bonds          
Fair Value Measurements          
Investments 6,915,943   6,915,943   18,182,542
Recurring | Level 2 | US Treasury bonds          
Fair Value Measurements          
Investments 6,016,914   6,016,914    
Recurring | Level 2 | Corporate bonds          
Fair Value Measurements          
Investments 71,221,192   71,221,192   63,997,742
Recurring | Level 3 | Institutional money market          
Fair Value Measurements          
Cash equivalents 0   0   0
Recurring | Level 3 | Certificates of deposit          
Fair Value Measurements          
Investments 0   0   0
Recurring | Level 3 | Municipal bonds          
Fair Value Measurements          
Investments 0   0   0
Recurring | Level 3 | Government agency bonds          
Fair Value Measurements          
Investments 0   0   0
Recurring | Level 3 | US Treasury bonds          
Fair Value Measurements          
Investments 0   0    
Recurring | Level 3 | Corporate bonds          
Fair Value Measurements          
Investments 0   0   0
Municipal bonds          
Held-to-maturity Securities          
Held-to-maturity securities, current 10,468,782   10,468,782   11,341,249
Held-to-maturity securities, noncurrent 2,292,620   2,292,620   0
Government agency bonds          
Held-to-maturity Securities          
Held-to-maturity securities, current 3,415,943   3,415,943   11,950,738
Held-to-maturity securities, noncurrent 3,500,000   3,500,000   6,231,804
US Treasury bonds          
Held-to-maturity Securities          
Held-to-maturity securities, current 6,016,914   6,016,914   0
Held-to-maturity securities, noncurrent 0   0   0
Corporate bonds          
Held-to-maturity Securities          
Held-to-maturity securities, current 57,296,635   57,296,635   57,321,784
Held-to-maturity securities, noncurrent 13,924,557   13,924,557   6,675,958
Certificates of deposit          
Held-to-maturity Securities          
Held-to-maturity securities, current 4,982,510   4,982,510   3,626,147
Held-to-maturity securities, noncurrent $ 1,074,101   $ 1,074,101   $ 3,661,262