Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenues by Product Group:
Total revenues	$ 138,466	$ 139,293	$ 276,075	$ 271,343
Corporate (non-product)
Revenues by Product Group:
Total revenues	278	243	576	463
Electronic Brokerage
Revenues by Product Group:
Total revenues	73,193	74,715	146,076	144,363
Research Sales and Trading
Revenues by Product Group:
Total revenues	30,311	28,140	59,556	53,553
Platforms
Revenues by Product Group:
Total revenues	23,333	24,595	47,066	49,694
Analytics
Revenues by Product Group:
Total revenues	$ 11,351	$ 11,600	$ 22,801	$ 23,270