Significant Accounting Policies - Company's Nonconsolidated VIEs (Detail) (Securitized HECM loan inventory [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2013
Securitized HECM loan inventory [Member]
Carrying value of the variable interests
Assets	$ 0
Liabilities	1,326
Maximum exposure to loss in nonconsolidated VIEs
Commitments	43,932
Purchased interests	$ 0

Variable interest entity entity maximum loss exposure commitments amount.

No definition available.

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Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Variable interest entity entity maximum loss exposure purchased interests amount.

+ References

No definition available.

+ Details

Name:	kcg_VariableInterestEntityEntityMaximumLossExposurePurchasedInterestsAmount
Namespace Prefix:	kcg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 810

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph a

-URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5728-111685

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of the liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration