Net Capital Requirements - Additional Information (Detail) $ in Millions	Mar. 31, 2018 USD ($)
Brokers And Dealers [Abstract]
Aggregate net capital and financial resources in excess of required level	$ 137.9
Aggregate net capital and financial resources, minimum capital requirement	$ 15.6

Amount of net capital requirement computed under the alternative method allowed under regulatory reporting requirements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

Name:	us-gaap_AlternativeNetCapitalRequirement1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant