Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted (Loss) Earnings Per Computations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Earnings Per Share [Abstract]
Net (loss) income attributable to common stockholders and shares used in basic calculations	$ (23,629)		$ 4,756		$ (11,297)		$ 35,906
(Loss) Income and shares used in diluted calculations	$ (23,629)		$ 4,756		$ (11,297)		$ 35,906
Shares used in basic calculations	359,955	[1]	89,624	[1]	306,879	[1]	89,685	[1]
Effect of dilutive stock based awards	0		3,058		0		3,482
(Loss) Income and shares used in diluted calculations	359,955	[1]	92,682	[1]	306,879	[1]	93,167	[1]
Basic earnings per share from continuing operations	$ (0.07)	[1]	$ 0.05	[1]	$ (0.04)	[1]	$ 0.40	[1]
Diluted earnings per share from continuing operations	$ (0.07)	[1]	$ 0.05	[1]	$ (0.04)	[1]	$ 0.39	[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