Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues
Commissions and fees	$ 391,419	$ 376,673	$ 437,022
Trading revenues, net	668,003	803,181	837,357
Expenses
Execution and clearance fees	295,312	265,186	305,177
Communications and data processing	147,986	139,263	150,595
Payment for order flow	54,765	61,741	70,183
Collateralized financing interest	40,423	34,678	27,860
Professional fees	19,827	27,055	25,596
Other expense	35,346	34,839	36,149
Assets
Securities borrowed	1,688,222	1,636,284
Receivable from brokers, dealers and clearing organizations	832,785	681,211
Other assets	164,168	222,831
Liabilities
Securities loaned	372,631	463,377
Payable to brokers, dealers and clearing organizations	518,900	273,805
Related Parties
Revenues
Commissions and fees	31,716	15,844	14,528
Trading revenues, net	1,839	6,468	3,862
Interest, net	581	914	651
Total revenues from related parties	34,136	23,226	19,041
Expenses
Execution and clearance fees	12,464	(15,472)	(10,261)
Communications and data processing	18,613	6,351	0
Payment for order flow	5	2,685	585
Collateralized financing interest	237	399	529
Professional fees	0	5,507	0
Other expense	231	2,349	1,719
Total expenses incurred with respect to related parties	31,550	1,819	$ (7,428)
Assets
Securities borrowed	5,293	10,573
Receivable from brokers, dealers and clearing organizations	2,106	1,987
Other assets	62,906	67,652
Liabilities
Securities loaned	2,594	3,844
Payable to brokers, dealers and clearing organizations	188	61
Accrued expenses and other liabilities	$ 3,708	$ 4,159