Receivables from and Payables to Broker-dealers, Clearing Organizations and Customers - Schedule of Receivables from and Payables to Broker-dealers, Clearing Organizations and Customers (Detail) $ in Thousands	Sep. 30, 2020 USD ($)
Receivables from broker-dealers, clearing organizations and customers:
Securities failed-to-deliver - broker-dealers	$ 111,605
Securities failed-to-deliver - customers	184,214
Deposits with clearing organizations and broker-dealers	76,830
Other	8,204
Total	380,853
Payables to broker-dealers, clearing organizations and customers:
Securities failed-to-receive - broker-dealers	84,932
Securities failed-to-receive - customers	46,091
Other	7,097
Total	$ 138,120