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Fair value of financial instruments - Marketable securities portfolio (Details) - Debt securities (corporate and government) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Summary of marketable security portfolio    
Amortized Cost $ 271,533 $ 271,401
Net Unrealized Losses (1,668) (1,265)
Estimated Fair Value $ 269,865 $ 270,136
Minimum    
Summary of marketable security portfolio    
Contractual maturity dates 12 months  
Maximum    
Summary of marketable security portfolio    
Contractual maturity dates 18 months