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) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Receivables from broker-dealers, clearing organizations and customers:
Securities failed-to-deliver - broker-dealers	$ 216,916	$ 93,294
Securities failed-to-deliver - customers	250,016	87,685
Deposits with clearing organizations and broker-dealers	80,862	97,043
Other	4,047	1,893
Total	551,841	279,915
Payables to broker-dealers, clearing organizations and customers:
Securities failed-to-receive - broker-dealers	184,755	70,917
Securities failed-to-receive - customers	110,930	60,784
Other	7,754	1,625
Total	$ 303,439	$ 133,326