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Cash equivalents and marketable securities (Tables)
12 Months Ended
Dec. 31, 2015
Cash and Cash Equivalents [Abstract]  
Schedule of Marketable Securities
Marketable securities at December 31, 2015 consisted of the following:
 
Amortized Cost
 
Unrealized Gains
 
Unrealized Losses
 
Fair Value
Current (due within 1 year):
 
 
 
 
 
 
 
Corporate debt securities
$
24,666,607

 
$
1,878

 
$
(16,137
)
 
$
24,652,348

Noncurrent (due after 1 year through 5 years):
 
 
 
 
 
 
 
Corporate debt securities
2,323,344

 

 
(10,395
)
 
2,312,949

Total
$
26,989,951

 
$
1,878

 
$
(26,532
)
 
$
26,965,297