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
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Brokers And Dealers [Line Items]      
Receivables from Brokers-Dealers and Clearing Organizations $ 8,464 $ 7,395  
Broker -Dealers And clearing Organisation [Member]      
Brokers And Dealers [Line Items]      
Receivables from Brokers-Dealers and Clearing Organizations   3,200 $ 7,400
Allowance for Doubtful Accounts Receivable   600 500
Broker -Dealers And clearing Organisation [Member] | Other Receivable [Member]      
Brokers And Dealers [Line Items]      
Accounts Receivable, Net   $ 4,900 $ 5,200