Quarterly report pursuant to Section 13 or 15(d)

Broker-Dealers and Clearing Organizations, Other Receivables and Prepaid Expenses and Other Current Liabilities - Additional Information (Detail)

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Broker-Dealers and Clearing Organizations, Other Receivables and Prepaid Expenses and Other Current Liabilities - Additional Information (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Brokers And Dealers [Line Items]      
Receivables from Brokers-Dealers and Clearing Organizations $ 8,282   $ 3,357
Broker -Dealers And clearing Organisation [Member]      
Brokers And Dealers [Line Items]      
Receivables from Brokers-Dealers and Clearing Organizations   $ 2,800  
Allowance for Doubtful Accounts Receivable   500  
Broker -Dealers And clearing Organisation [Member] | Other Receivable [Member]      
Brokers And Dealers [Line Items]      
Accounts Receivable, Net   $ 6,000