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Investments - Schedule of Information About Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Short-term      
Investments short term $ 705,108 $ 483,002 $ 480,233
Long-term      
Investments long term 65,953 953 $ 861
Time deposits between 90 and 365 days [member]      
Short-term      
Time deposits between 90 and 365 days 705,108 483,002  
Time deposits of more than 365 days [member]      
Long-term      
Time deposits of more than 365 days $ 65,953 $ 953