Segment Reporting - Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Total revenues	$ 224,173	$ 120,409	$ 140,494	$ 149,727	$ 148,966	$ 134,773	$ 138,466	$ 137,609	$ 634,803	$ 559,814	$ 530,801
Corporate
Revenues
Total revenues									108,637	1,305	1,458
Electronic Brokerage
Revenues
Total revenues									268,818	294,529	279,829
Research Sales and Trading
Revenues
Total revenues									117,340	122,042	107,383
Platforms
Revenues
Total revenues									94,117	95,926	96,127
Analytics
Revenues
Total revenues									$ 45,891	$ 46,012	$ 46,004