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Marketable Securities (Tables)
 3 Months Ended
Aug. 31, 2017
Investments, Debt and Equity Securities [Abstract]  
Marketable Securities
As of August 31, 2017 and May 31, 2017, marketable securities consisted of the following (in thousands of dollars):
 
 
August 31, 2017
(in thousands)
Amortized
cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Fair Value
Available-for-sale securities:
 
 
 
 
 
 
 
Government agency obligations
$
1,350

 
$
—

 
$
(135
)
 
$
1,215

 
$
1,350

 
$
—

 
$
(135
)
 
$
1,215

 
 
May 31, 2017
(in thousands)
Amortized
cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Fair Value
Available-for-sale securities:
 
 
 
 
 
 
 
Government agency obligations
$
1,350

 
$
—

 
$
(135
)
 
$
1,215

 
$
1,350

 
$
—

 
$
(135
)
 
$
1,215