Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Revenues
Commissions and fees	$ 87,842	$ 95,027	$ 288,904	$ 282,358
Trading revenues, net	113,829	277,677	524,649	657,222
Expenses
Execution and clearance fees	71,995	67,502	219,371	198,573
Communications and data processing	36,733	35,256	108,766	103,260
Payment for order flow	13,845	17,121	39,590	47,277
Collateralized financing interest	10,693	8,617	29,465	25,932
Professional fees	4,139	4,406	15,497	21,281
Other expense	8,797	10,000	25,923	26,604
Assets
Securities borrowed	1,958,095		1,958,095		$ 1,636,284
Receivable from brokers, dealers and clearing organizations	696,495		696,495		681,211
Other assets	194,072		194,072		222,831
Liabilities
Securities loaned	647,602		647,602		463,377
Payable to brokers, dealers and clearing organizations	647,089		647,089		273,805
Related Parties
Revenues
Commissions and fees	8,171	4,496	23,246	10,624
Trading revenues, net	86	1,343	1,190	5,248
Interest, net	119	399	394	756
Total revenues from related parties	8,376	6,238	24,830	16,628
Expenses
Execution and clearance fees	4,132	(4,525)	8,577	(13,333)
Communications and data processing	4,710	1,577	12,327	3,986
Payment for order flow	0	274	5	2,527
Collateralized financing interest	73	99	199	346
Professional fees	0	0	0	5,507
Other expense	71	479	125	1,756
Total expenses incurred with respect to related parties	8,986	$ (2,096)	21,233	$ 789
Assets
Securities borrowed	35,421		35,421		10,573
Receivable from brokers, dealers and clearing organizations	4,601		4,601		1,987
Other assets	62,789		62,789		67,652
Liabilities
Securities loaned	3,127		3,127		3,844
Payable to brokers, dealers and clearing organizations	61		61		61
Accrued expenses and other liabilities	$ 4,016		$ 4,016		$ 4,159