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Marketable Securities (Details) (USD $)
In Thousands, unless otherwise specified
Nov. 30, 2012
May 31, 2012
Marketable securities    
Amortized cost $ 2,154 $ 14,255
Gross Unrealized Gains 1 15
Gross Unrealized Losses    (200)
Fair Value 2,155 14,070
U.S. government agency obligations [Member]
   
Marketable securities    
Amortized cost 1,850 7,739
Gross Unrealized Gains    5
Gross Unrealized Losses    (45)
Fair Value 1,850 7,699
Corporate bond securities [Member]
   
Marketable securities    
Amortized cost 304 6,516
Gross Unrealized Gains 1 10
Gross Unrealized Losses    (155)
Fair Value $ 305 $ 6,371