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	Mar. 31, 2021	Dec. 31, 2020
Receivables from broker-dealers, clearing organizations and customers:
Securities failed-to-deliver - broker-dealers	$ 155,693	$ 93,294
Securities failed-to-deliver - customers	288,235	87,685
Deposits with clearing organizations and broker-dealers	111,594	97,043
Other	9,435	1,893
Total	564,957	279,915
Payables to broker-dealers, clearing organizations and customers:
Securities failed-to-receive - broker-dealers	156,322	70,917
Securities failed-to-receive - customers	144,958	60,784
Other	6,988	1,625
Total	$ 308,268	$ 133,326