Revenue from Contracts with Customers - Information about Receivables with Customers (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Other Assets [Member]
Disaggregation of Revenue [Line Items]
Receivables, which are included in "Other assets"	$ 1,348	$ 1,397

Amount due from customers for fees and charges arising from transactions related to the entity's brokerage activities and operations.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 310
-URI http://asc.fasb.org/subtopic&trid=2176284

Name:	us-gaap_ReceivablesFromCustomers
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant