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Fair value of financial instruments and marketable securities - Available for sale (Details) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Marketable securities accounted for as available-for-sale securities    
Amortized Cost $ 236,962 $ 281,492
Gross Unrealized Gains 299 83
Gross Unrealized Losses (26) (672)
Marketable securities 237,235 280,903
Transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy 0 0
Commercial paper    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 11,969 26,877
Gross Unrealized Gains 21 80
Marketable securities 11,990 26,957
Corporate debt securities    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 210,640 226,959
Gross Unrealized Gains 274  
Gross Unrealized Losses (26) (640)
Marketable securities 210,888 226,319
Government obligations    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 14,353 27,656
Gross Unrealized Gains 4 3
Gross Unrealized Losses   (32)
Marketable securities $ 14,357 $ 27,627