Acquisitions (Details) (Penson Futures [Member], USD $) In Millions, unless otherwise specified	Jun. 01, 2013
Penson Futures [Member]
Business Acquisition [Line Items]
Pension Futures Acquired	$ 5.0
Pension Potential Payout Acquired	1.0
Goodwill and intangible assets acquired	$ 5.9

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Goodwill and Intangible Assets

No definition available.

Name:	kcg_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedGoodwillandIntangibleAssets
Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant