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Financial Instruments - Exposure to Credit Risk (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of credit risk exposure [line items]      
Cash and short-term investments $ 138,510 $ 175,953 $ 180,881
Short-term investments 74,004 51,590  
Credit risk [member]      
Disclosure of credit risk exposure [line items]      
Cash and short-term investments 138,510 175,953  
Short-term investments 74,004 51,590  
Trade accounts receivable 40,803 51,952  
Royalty receivable 105 60  
Employee loans $ 312 $ 491