Significant Clients - Additional Information (Detail)	12 Months Ended
Dec. 31, 2014 Client	Dec. 31, 2013 Client	Dec. 31, 2012 Client
Risks and Uncertainties [Abstract]
Number of significant clients	0kcg_NumberOfSignificantClients	0kcg_NumberOfSignificantClients	0kcg_NumberOfSignificantClients
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