Net Capital Requirements - Financial Resource Requirements on Significant Foreign Regulated Broker-Dealer (Details) (USD $)	12 Months Ended
Dec. 31, 2014
Net Capital Requirements [Line Items]
Release of excess capital	$ 45,000,000kcg_ReleaseofExcessNetCapital
KCG Europe Limited [Member]
Net Capital Requirements [Line Items]
Net Capital	187,940,000
Resource Requirement	(124,577,000)
Excess Financial Resources	63,363,000
GETCO Europe Limited [Member]
Net Capital Requirements [Line Items]
Net Capital	7,254,000
Resource Requirement	(1,759,000)
Excess Financial Resources	$ 5,495,000

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Release of Excess Net Capital

+ References

No definition available.

+ Details

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- Definition

The amount of net capital in excess of the alternative minimum required net capital.

+ References

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

-Publisher SEC

-Name Rule 15c3

-Number 240

-Section 1

-Paragraph 1

-Subparagraph ii

-Article 15c3

-Subsection a

+ Details

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- Definition

Amount of net capital requirement computed under the alternative method allowed under regulatory reporting requirements.

+ References

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

-Publisher SEC

-Name Rule 15c3

-Number 240

-Section 1

-Paragraph 1

-Subparagraph ii

-Article 15c3

-Subsection a

+ Details

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- Definition

Net capital is equal to the net worth of the broker dealer, less certain items such as exchange memberships, carrying value of securities not readily marketable, haircuts on marketable securities in proprietary accounts, furniture and equipment and other illiquid assets as defined.

+ References

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

-Publisher SEC

-Name Rule 15c3

-Number 240

-Section 1

-Paragraph 1

-Subparagraph ii

-Article 15c3

-Subsection a

+ Details

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