Collateralized Transaction - Additional Information (Detail)	Sep. 30, 2013
Collateralized Agreements [Abstract]
Repurchase agreements and other secured financings, maturity (years)	within one year

Repurchase agreements and other secured financings maturity period.

No definition available.

Name:	kcg_RepurchaseAgreementsAndOtherSecuredFinancingsMaturityPeriod
Namespace Prefix:	kcg_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	instant