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Accounts Receivable - Provision for Impairment of Receivables (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of accounts receivables [abstract]      
Beginning balance $ (3,739) $ (2,997) $ (3,691)
(Additions) reversals (879) (1,511) 71
Write-offs 945 769 623
Ending balance $ (3,673) $ (3,739) $ (2,997)