Goodwill and Intangible Assets - Summary of Goodwill (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Goodwill	$ 16,404	$ 17,311
Operating Segments \| Market Making
Goodwill [Line Items]
Goodwill	16,404	16,404
Operating Segments \| Global Execution Services
Goodwill [Line Items]
Goodwill	$ 0	$ 907

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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