v3.8.0.1
Financial Instruments - Schedule of Financial Instruments, Measured at Fair Value on a Recurring and Non-recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents $ 141,677 $ 84,013 $ 110,070
Level 1 | U.S. federal bonds      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 119,057 70,667  
Level 1 | Canadian federal bonds      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 19,940    
Level 1 | U.S. term deposits      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash equivalents 65,284 46,385  
Level 2 | Foreign exchange forward contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative Asset 4,503    
Derivatives 795 1,818  
Level 2 | Corporate bonds and commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Marketable securities 593,554 191,345  
Level 2 | Corporate bonds and commercial paper      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents $ 9,965    
Level 2 | U.S. federal bonds      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents   $ 9,994