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Marketable Securities (Tables)
 6 Months Ended
Nov. 30, 2015
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities
As of November 30, 2015 and May 31, 2015, marketable securities consisted of the following (in thousands of dollars):
 
As of November 30, 2015
Amortized
cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Fair Value
Available-for-sale securities
 
 
 
 
 
 
 
U.S. government agency obligations
$
1,800

 
$
—

 
$
(161
)
 
$
1,639

 
$
1,800

 
$
—

 
$
(161
)
 
$
1,639

 
As of May 31, 2015
Amortized
cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Fair Value
Available-for-sale securities
 
 
 
 
 
 
 
U.S. government agency obligations
$
1,825

 
$
—

 
$
(136
)
 
$
1,689

 
$
1,825

 
$
—

 
$
(136
)
 
$
1,689