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Cash equivalents and marketable securities (Tables)
 12 Months Ended
Dec. 31, 2018
Cash and Cash Equivalents [Abstract]  
Schedule of marketable securities
Marketable securities at December 31, 2017 consisted of the following:
 

 
Amortized Cost
 
Unrealized Gains
 
Unrealized Losses
 
Fair Value
As of December 31, 2017
 
 
 
 
 
 
 
Current (due within 1 year):
 
 
 
 
 
 
 
U.S. government agency securities
$
8,987,254

 
$
38

 
$
(1,033
)
 
$
8,986,259

Commercial paper
4,440,689

 
—

 
—

 
4,440,689

    Corporate debt securities
703,027

 
—

 
(252
)
 
702,775

Total
$
14,130,970

 
$
38

 
$
(1,285
)
 
$
14,129,723