Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Summary of Components of Accumulated Other Comprehensive Income (Loss) (Details)

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Summary of Significant Accounting Policies - Summary of Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]      
Beginning Balance $ (35) $ (37)  
Other comprehensive gain (loss) 307 2 $ (37)
Ending Balance 272 (35) $ (37)
Foreign Currency Translation Adjustments      
Accumulated Other Comprehensive Income Loss [Line Items]      
Other comprehensive gain (loss) 363    
Ending Balance 363    
Unrealized Gains (Losses) on Available-for-sale Investments      
Accumulated Other Comprehensive Income Loss [Line Items]      
Beginning Balance (82)    
Other comprehensive gain (loss) 65    
Ending Balance (17) (82)  
Unrealized Gains (Losses) on Cash Flow Hedges      
Accumulated Other Comprehensive Income Loss [Line Items]      
Beginning Balance 47    
Other comprehensive gain (loss) (121)    
Ending Balance $ (74) $ 47