Collateralized Transactions - Additional Information (Details)	9 Months Ended
Sep. 30, 2016
Collateralized Agreements [Abstract]
Repurchase agreements and other secured financings, maturity (years)	1 year

Repurchase agreements and other secured financings maturity period.

No definition available.

Name:	kcg_RepurchaseAgreementsandOtherSecuredFinancingsMaturityPeriod
Namespace Prefix:	kcg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration