Net Capital Requirements - Additional Information (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Brokers And Dealers [Abstract]
Aggregate net capital and financial resources in excess of required level	$ 130.5
Aggregate net capital and financial resources, minimum capital requirement	$ 14.8

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

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