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Cash and cash equivalents - Summary of Cash and Cash Equivalents (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Cash and cash equivalents [abstract]      
Cash at bank and on hand $ 200,644 $ 293,437  
Short-term bank deposits 85,668 186,064  
Cash and cash equivalents $ 286,312 $ 479,501 $ 883,171