Business Held for Sale and Discontinued Operations - Income Disclosures (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
Expenses:
Compensation	$ 135,992	[1]	$ 100,964	[1]	$ 243,070	[1]	$ 212,412	[1]
Other	24,090	[1]	8,924	[1]	30,491	[1]	14,366	[1]
Total expenses	337,165	[1]	244,485	[1]	603,005	[1]	494,427	[1]
(Loss) income from discontinued operations, net of tax	(7,213)	[1]	(1,465)	[1]	(28,901)	[1]	491	[1]
Segment, Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenues	37,613		37,159		67,345		85,253
Expenses:
Compensation	38,028		29,914		68,934		65,721
Other	9,168		9,499		19,856		18,776
Writedown of assets and lease loss accrual, net	1,514		0		23,018		0
Total expenses	48,710		39,413		111,808		84,497
Pre-tax (loss) income from discontinued operations	(11,097)		(2,254)		(44,463)		756
Income tax benefit (expense)	3,884		789		15,562		(265)
(Loss) income from discontinued operations, net of tax	$ (7,213)		$ (1,465)		$ (28,901)		$ 491

[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