Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Revenues
Commissions and fees	$ 87,370	$ 104,776	$ 187,331	$ 217,033
Trading revenues, net	170,750	206,780	379,545	465,077
Expenses
Execution and clearance fees	62,598	73,242	131,071	148,743
Communications and data processing	34,240	38,279	68,004	75,075
Payment for order flow	14,935	18,076	30,156	40,108
Collateralized financing interest	8,859	6,395	17,315	12,557
Professional fees	5,694	7,337	16,875	12,739
Other expense	10,652	10,767	18,460	19,410
Assets
Securities borrowed	1,871,312		1,871,312		$ 1,632,062
Receivable from brokers, dealers and clearing organizations	690,291		690,291		1,188,833
Other assets	234,459		234,459		137,645
Liabilities
Securities loaned	741,732		741,732		707,744
Payable to brokers, dealers and clearing organizations	529,748		529,748		676,089
Related Parties [Member]
Revenues
Commissions and fees	2,636	2,740	6,128	7,886
Trading revenues, net	1,804	541	3,905	1,355
Interest, net	131	130	357	318
Total revenues from related parties	4,571	3,411	10,390	9,559
Expenses
Execution and clearance fees	(4,180)	(2,537)	(8,808)	(6,548)
Communications and data processing	1,299	0	2,409	0
Payment for order flow	1,073	0	2,253	0
Collateralized financing interest	134	120	247	279
Professional fees	0	0	5,507	0
Other expense	656	434	1,277	834
Total expenses incurred with respect to related parties	(1,018)	$ (1,983)	2,885	$ (5,435)
Assets
Securities borrowed	45,466		45,466		26,110
Receivable from brokers, dealers and clearing organizations	107,946		107,946		20,075
Other assets	67,167		67,167		0
Liabilities
Securities loaned	1,267		1,267		7,376
Payable to brokers, dealers and clearing organizations	40,128		40,128		8,509
Accrued expenses and other liabilities	$ 4,626		$ 4,626		$ 5,667