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Merger of GETCO and Knight - Intangible Assets Acquired (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Jul. 01, 2013
Business Acquisition [Line Items]      
Goodwill $ 17,311us-gaap_Goodwill $ 17,311us-gaap_Goodwill  
Weighted average useful life 5 years 6 years  
Knight [Member]      
Business Acquisition [Line Items]      
Intangible assets     155,425us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill
/ us-gaap_BusinessAcquisitionAxis
= kcg_KnightMember
Goodwill     12,666us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= kcg_KnightMember
Total     168,091us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles
/ us-gaap_BusinessAcquisitionAxis
= kcg_KnightMember
Knight [Member] | Technology [Member]      
Business Acquisition [Line Items]      
Intangible assets     110,504us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill
/ us-gaap_BusinessAcquisitionAxis
= kcg_KnightMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_DevelopedTechnologyRightsMember
Weighted average useful life     5 years
Knight [Member] | Customer Relationships [Member]      
Business Acquisition [Line Items]      
Intangible assets     35,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill
/ us-gaap_BusinessAcquisitionAxis
= kcg_KnightMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_CustomerRelationshipsMember
Knight [Member] | Customer Relationships [Member] | Minimum [Member]      
Business Acquisition [Line Items]      
Weighted average useful life     9 years
Knight [Member] | Customer Relationships [Member] | Maximum [Member]      
Business Acquisition [Line Items]      
Weighted average useful life     11 years
Knight [Member] | Trade Names [Member]      
Business Acquisition [Line Items]      
Intangible assets     4,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill
/ us-gaap_BusinessAcquisitionAxis
= kcg_KnightMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember
Weighted average useful life     10 years
Knight [Member] | Trading Rights [Member]      
Business Acquisition [Line Items]      
Intangible assets     $ 5,921us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill
/ us-gaap_BusinessAcquisitionAxis
= kcg_KnightMember
/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis
= kcg_TradingRightsMember		 [1]
Weighted average useful life     7 years [1]
[1] Trading rights include both assets with a finite useful life and assets with an indefinite useful life. The 7 years amortization period only applies to assets with a finite useful life.