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Accounts receivable - Additional Information (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Allowance for doubtful accounts receivable $ 5,281 $ 5,555
Unbilled revenue    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accounts receivable balances $ 79,742 $ 78,289