Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investments [Abstract]
Investments accounted for under the equity method	$ 86,328	$ 110,460
Investments held at fair value	4,435us-gaap_AvailableForSaleSecurities	3,925us-gaap_AvailableForSaleSecurities
Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	8,949	8,953
Total Strategic investments	99,712kcg_StrategicInvestments	123,338kcg_StrategicInvestments
Deferred compensation investments	868us-gaap_DeferredCompensationPlanAssets	117us-gaap_DeferredCompensationPlanAssets
Investment in Deephaven Funds	146kcg_InvestmentInFunds	1,958kcg_InvestmentInFunds
Total Investments	$ 100,726us-gaap_Investments	$ 125,413us-gaap_Investments

Investment in funds.

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- Definition

Private companies investments less than twenty percentage equity ownership held at adjusted cost.

+ References

No definition available.

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- Definition

Strategic Investments.

+ References

No definition available.

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

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- Definition

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

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-Name Regulation S-X (SX)

-Number 210

-Section 02

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- Definition

Sum of the carrying amounts as of the balance sheet date of all investments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

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