Segment Reporting (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Revenues
Total revenues	$ 149,727	$ 137,609
Corporate
Revenues
Total revenues	233	297
Electronic Brokerage
Revenues
Total revenues	80,454	72,884
Research Sales and Trading
Revenues
Total revenues	32,513	29,245
Platforms
Revenues
Total revenues	25,073	23,733
Analytics
Revenues
Total revenues	$ 11,454	$ 11,450