Significant Clients - Additional Information (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2013 Client	Sep. 30, 2012 Client	Sep. 30, 2013 Client	Sep. 30, 2012 Client
Risks and Uncertainties [Abstract]
Number of significant clients	0	0	0	0
Percentage of dollar value traded	10.00%	10.00%		10.00%

Stated minimum percentage of domestic equity dollar value traded for significant client consideration.

No definition available.

Name:	kcg_StatedMinimumPercentageOfDomesticEquityDollarValueTradedForSignificantClientConsideration
Namespace Prefix:	kcg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration