Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Revenues
Commissions and fees	$ 93,589	$ 106,101
Trading revenues, net	154,307	223,938
Expenses
Execution and clearance fees	72,795	73,634
Communications and data processing	39,020	35,657
Payment for order flow	17,121	12,655
Collateralized financing interest	11,761	9,163
Other expense	8,197	9,201
Assets
Securities borrowed	1,594,442		$ 1,688,222
Receivable from brokers, dealers and clearing organizations	801,678		832,785
Other assets	192,359		164,168
Liabilities
Securities loaned	459,533		372,631
Payable to brokers, dealers and clearing organizations	495,253		518,900
Related Parties
Revenues
Commissions and fees	1,868	6,663
Trading revenues, net	791	477
Interest, net	119	103
Total revenues from related parties	2,778	7,243
Expenses
Execution and clearance fees	313	880
Communications and data processing	5,610	3,206
Payment for order flow	0	5
Collateralized financing interest	86	78
Other expense	0	5
Total expenses incurred with respect to related parties	6,009	$ 4,174
Assets
Securities borrowed	17,754		5,293
Receivable from brokers, dealers and clearing organizations	2,134		2,106
Other assets	16		62,906
Liabilities
Securities loaned	2,063		2,594
Payable to brokers, dealers and clearing organizations	45		188
Accrued expenses and other liabilities	$ 122		$ 3,708