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Financial Instruments - Exposure to Credit Risk (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of credit risk exposure [line items]      
Cash and short-term investments $ 175,953 $ 180,881 $ 133,963
Short-term investments 51,590 36,729  
Credit risk [member]      
Disclosure of credit risk exposure [line items]      
Cash and short-term investments 175,953 180,881  
Short-term investments 51,590 36,729  
Trade receivables 51,952 44,960  
Royalty receivable 60 20  
Employee Loans Receivable $ 491 $ 1,048