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Cash and Cash Equivalents - Summary of Cash and Cash Equivalents (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash and cash equivalents [abstract]      
Cash at bank and on hand $ 293,437 $ 282,021  
Short-term bank deposits 186,064 601,150  
Cash and cash equivalents $ 479,501 $ 883,171 $ 288,189