Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Revenues
Commissions and fees	$ 126,169	[1]	$ 139,838	[1]	$ 250,546	[1]	$ 279,599	[1]
Expenses
Execution and clearance fees	51,518	[1]	52,276	[1]	101,881	[1]	104,493	[1]
Assets
Securities borrowed	1,158,981				1,158,981				1,008,720
Receivable from brokers, dealers and clearing organizations	1,366,974				1,366,974				868,805
Other assets	253,847				253,847				251,773
Liabilities
Securities loaned	626,891				626,891				504,082
Payable to brokers, dealers and clearing organizations	635,914				635,914				378,724
Affiliated entity [Member]
Revenues
Commissions and fees	587				773
Interest Revenue (Expense), Net	86				86
Total revenues from related parties	673				859
Expenses
Other expense	27				27
Execution and clearance fees	2,514				2,525
Interest Expense, Related Party	222				222
Total expenses incurred with respect to related parties	2,763				2,774
Assets
Securities borrowed	21,540				21,540				42,196
Receivable from brokers, dealers and clearing organizations	184				184				728
Other assets	1,535				1,535				1,608
Liabilities
Securities loaned	4,199				4,199				3,754
Payable to brokers, dealers and clearing organizations	84				84				57
Accrued expenses and other liabilities	$ 2,180				$ 2,180				$ 359

[1] See Explanatory Note - All financial information is of Knight Capital Group, Inc. As of July 1, 2013, Knight Capital Group, Inc., became a wholly-owned subsidiary of KCG