Securities Owned and Securities Sold, Not Yet Purchased - Additional Information (Detail) - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Aggregate securities owned	$ 318,580,000	$ 32,361,000
Total securities sold, not yet purchased	$ 0	$ 0

Fair value of financial instruments sold short by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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