Net Capital Requirements - Net Capital Levels and Requirements for U.S. Registered Broker-Dealer Subsidiary (Details) (KCG Americas LLC [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
KCG Americas LLC [Member]
Net Capital And Net Capital Requirements For Companys Broker Dealer Subsidiaries [Line Items]
Net Capital	$ 285,231
Net Capital Requirement	1,000
Excess Financial Resources	$ 284,231

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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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