Significant Accounting Policies - Company's Nonconsolidated VIEs (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Securitized HECM loan inventory [Member]
Carrying value of the variable interests
Assets	$ 0	$ 0
Liabilities	1,215	912
Maximum exposure to loss in nonconsolidated VIEs
Commitments	39,053	29,017
Purchased interests	0	0
MBS held within Assets of business held for sale [Member]
Carrying value of the variable interests
Assets	0	108,999
Liabilities	0	0
Maximum exposure to loss in nonconsolidated VIEs
Commitments	0	0
Purchased interests	$ 0	$ 108,999

Variable interest entity entity maximum loss exposure commitments amount.

No definition available.

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Period Type:	instant

- Definition

Variable interest entity entity maximum loss exposure purchased interests amount.

+ References

No definition available.

+ Details

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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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- Details

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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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- Details

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