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Marketable Securities (Tables)
 3 Months Ended
Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract]  
Fair value of marketable securities, by type The following table summarizes the fair value of our marketable securities, by level within the fair value hierarchy, as of each period end:
March 31, 2018 December 31, 2017 
Level 1 Level 2 Total Level 1 Level 2 Total 
U.S. Treasury notes and bonds $6,486,345 $— $6,486,345 $— $— $— 
U.S. Agency issued mortgage-backed securities – variable rate $— $1,569,884 $1,569,884 $— $3,539,102 $3,539,102 
U.S. Agency notes and bonds – fixed rate — — — — 198,293 198,293 
Corporate bonds — 2,502,977 2,502,977 — — — 
SBA loan pools – variable rate — 548,641 548,641 — 935,081 935,081 
Short-term cash investments — 4,502,258 4,502,258 — — — 
Total fair value of marketable securities $6,486,345 $9,123,760 $15,610,105 $— $4,672,476 $4,672,476