Significant Accounting Policies - Schedule of Interest Income and Interest Expense (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
Accounting Policies [Abstract]
Interest Income	$ 58,416		$ 31,337		$ 111,777		$ 60,675
Interest	(52,603)		(27,110)		(99,575)		(50,838)
Interest, net	$ 5,813	[1]	$ 4,227	[1]	$ 12,202	[1]	$ 9,837	[1]

[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