Segment Reporting (Detail 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Total revenues	$ 148,966	$ 134,773	$ 138,466	$ 137,609	$ 131,900	$ 127,558	$ 139,293	$ 132,050	$ 559,814	$ 530,801	$ 504,436
Corporate (non-product)
Revenues
Total revenues									1,305	1,457	1,285
Electronic Brokerage
Revenues
Total revenues									294,529	279,830	250,882
Research Sales and Trading
Revenues
Total revenues									122,042	107,383	106,427
Platforms
Revenues
Total revenues									95,926	96,127	99,334
Analytics
Revenues
Total revenues									$ 46,012	$ 46,004	$ 46,508