Regulatory Capital Requirements - Additional Information (Detail) $ in Millions	Sep. 30, 2020 USD ($)
Brokers And Dealers [Abstract]
Aggregate net capital and financial resources in excess of required level	$ 305.6
Aggregate net capital and financial resources, minimum capital requirement	23.7
Securities reserve deposit	$ 50.0

Minimum amount of net capital of broker-dealer and its subsidiary, calculated under Alternative Standard.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

Name:	srt_AlternativeNetCapitalRequirement1
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant