Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Commissions and fees	$ 376,673	$ 437,022	$ 275,474
Trading revenues, net	803,181	837,357	628,304
Expenses
Execution and clearance fees	265,186	305,177	246,414
Communications and data processing	139,263	150,595	123,552
Payment for order flow	61,741	70,183	35,711
Collateralized financing interest	34,678	27,860	9,847
Professional fees	27,055	25,596	46,662
Other expense	38,375	39,814	43,094
Assets
Securities borrowed	1,636,284	1,632,062
Receivable from brokers, dealers and clearing organizations	681,211	1,188,833
Other assets	233,474	137,645
Liabilities
Securities loaned	463,377	707,744
Payable to brokers, dealers and clearing organizations	273,805	676,089
Related Parties
Revenues
Commissions and fees	15,844	14,528	4,045
Trading revenues, net	6,468	3,862	437
Interest, net	914	651	71
Total revenues from related parties	23,226	19,041	4,553
Expenses
Execution and clearance fees	(15,472)	(10,261)	(19,491)
Communications and data processing	6,351	0	0
Payment for order flow	2,685	585	0
Collateralized financing interest	399	529	102
Professional fees	5,507	0	0
Other expense	2,349	1,719	651
Total expenses incurred with respect to related parties	1,819	(7,428)	$ (18,738)
Assets
Securities borrowed	10,573	26,110
Receivable from brokers, dealers and clearing organizations	1,987	20,075
Other assets	67,652	0
Liabilities
Securities loaned	3,844	7,376
Payable to brokers, dealers and clearing organizations	61	8,509
Accrued expenses and other liabilities	$ 4,159	$ 5,667