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Accounts Receivable and Allowance for Doubtful Accounts - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Allowance For Doubtful Accounts And Unbilled Services [Abstract]        
Provision for doubtful accounts $ 3,200 $ 4,500 $ 11,951 $ 10,510