Collateralized Transactions and Assets and Liabilities Subject to Netting - Additional Information (Detail)	12 Months Ended
Dec. 31, 2014
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