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Fair value of financial instruments and marketable securities - Available for sale (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 80,696 $ 79,432
Gross Unrealized Gains 30 52
Gross Unrealized Losses (131) (30)
Fair Value 80,595 79,454
Commercial paper    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 26,191 13,775
Gross Unrealized Gains 30 52
Gross Unrealized Losses 0 0
Fair Value 26,221 13,827
Corporate debt securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 54,505 65,657
Gross Unrealized Gains 0 0
Gross Unrealized Losses (131) (30)
Fair Value $ 54,374 $ 65,627