Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Revenues by product group
Total revenues	$ 132,050	$ 136,375
Corporate (non-product)
Revenues by product group
Total revenues	220	428
Electronic Brokerage
Revenues by product group
Total revenues	69,648	72,180
Research Sales and Trading
Revenues by product group
Total revenues	25,413	26,426
Platforms
Revenues by product group
Total revenues	25,099	25,744
Analytics
Revenues by product group
Total revenues	$ 11,670	$ 11,597