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Fair value of financial instruments and marketable securities (Details 2) - USD ($)
Dec. 31, 2016
Dec. 31, 2015
Marketable securities accounted for as available-for-sale securities    
Transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy $ 0  
Amortized Cost 173,347,000 $ 281,492,000
Gross Unrealized Gains 101,000 83,000
Gross Unrealized Losses (103,000) (672,000)
Fair Value 173,345,000 280,903,000
Commercial paper    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 12,919,000 26,877,000
Gross Unrealized Gains 47,000 80,000
Gross Unrealized Losses 0 0
Fair Value 12,966,000 26,957,000
Corporate debt securities    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 153,240,000 226,959,000
Gross Unrealized Gains 52,000 0
Gross Unrealized Losses (103,000) (640,000)
Fair Value 153,189,000 226,319,000
Government obligations    
Marketable securities accounted for as available-for-sale securities    
Amortized Cost 7,188,000 27,656,000
Gross Unrealized Gains 2,000 3,000
Gross Unrealized Losses 0 (32,000)
Fair Value $ 7,190,000 $ 27,627,000