Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues by product group
Total revenues	$ 127,558	$ 119,617	$ 398,901	$ 382,902
Corporate (non-product)
Revenues by product group
Total revenues	313	287	776	1,017
Electronic Brokerage
Revenues by product group
Total revenues	66,234	57,654	210,597	192,539
Research Sales and Trading
Revenues by product group
Total revenues	26,683	25,792	80,236	79,023
Platforms
Revenues by product group
Total revenues	23,151	24,188	72,845	75,672
Analytics
Revenues by product group
Total revenues	$ 11,177	$ 11,696	$ 34,447	$ 34,651