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Fair value of financial instruments and marketable securities (Details 2) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Marketable securities accounted for as available-for-sale securities    
Amortized Cost $ 231,777 $ 265,880
Gross Unrealized Gains 69 87
Gross Unrealized Losses (1,037) (474)
Fair Value 230,809 265,493
Corporate debt securities    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 198,102 230,379
Gross Unrealized Gains 52 80
Gross Unrealized Losses (987) (428)
Fair Value 197,167 230,031
Government obligations    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 33,675 35,501
Gross Unrealized Gains 17 7
Gross Unrealized Losses (50) (46)
Fair Value $ 33,642 $ 35,462