Segment Reporting - Revenue by Product Group (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Revenues
Total Revenues	$ 124,668	$ 149,727
Corporate
Revenues
Total Revenues	347	233
Electronic Brokerage
Revenues
Total Revenues	64,261	80,454
Research Sales and Trading
Revenues
Total Revenues	25,532	32,513
Platforms
Revenues
Total Revenues	23,593	25,073
Analytics
Revenues
Total Revenues	$ 10,935	$ 11,454

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Article 5

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration