XML 137 R69.htm IDEA: XBRL DOCUMENT v2.4.1.9
Derivative Financial Instruments - Fair Value of Derivative Instruments (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
contract
Dec. 31, 2013
contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value $ 169,423us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral $ 355,230us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
Assets, Contract 458,242us-gaap_DerivativeAssetNumberOfInstrumentsHeld 760,891us-gaap_DerivativeAssetNumberOfInstrumentsHeld
Liabilities, Fair Value 139,811us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral 269,078us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Liabilities, Contract 469,292us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld 776,537us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
Foreign Currency Futures Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Receivable from Brokers, Dealers and Clearing Organizations [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value 1,212us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 89us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Assets, Contract 8,108us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 892us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Fair Value 651us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 142us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Contract 9,090us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 533us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeFutureMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Foreign Currency Forward Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Financial Instruments Owned, at Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value 60us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		  
Assets, Contract 1us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		  
Liabilities, Fair Value 0us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		  
Liabilities, Contract 0us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		  
Foreign Currency Forward Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Other Assets [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value   6,913us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Assets, Contract   1us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Fair Value   6,501us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Contract   1us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Foreign Currency Forward Contracts [Member] | Derivative Instruments Designated as Hedging Instruments [Member] | Financial Instruments Sold, Not Yet Purchased, at Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value   0us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsSoldNotYetPurchasedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember		 [1]
Assets, Contract   0us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsSoldNotYetPurchasedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember		 [1]
Liabilities, Fair Value   5,014us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsSoldNotYetPurchasedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember		 [1]
Liabilities, Contract   1us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsSoldNotYetPurchasedatFairValueMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeForwardMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember		 [1]
Equity Futures Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Receivable from Brokers, Dealers and Clearing Organizations [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value 1,790us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 223us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Assets, Contract 2,590us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 1,069us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Fair Value 2,047us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 1,089us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Contract 3,085us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 1,046us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_EquityFutureContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Equity Swap Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Receivable from Brokers, Dealers and Clearing Organizations [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value 98us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Assets, Contract 1us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 0us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Fair Value 13us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 18us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Contract 1us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 1us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_EquitySwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Listed Options [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Financial Instruments Owned/Sold, Not Yet Purchased, at Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value 144,586us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 339,798us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Assets, Contract 426,747us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 730,020us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Fair Value 115,362us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 252,282us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Contract 437,383us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 755,947us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_FinancialInstrumentsOwnedSoldNotYetPurchasedatFairValueMemberMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ExchangeTradedOptionsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Fixed Income Futures Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Receivable from Brokers, Dealers and Clearing Organizations [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value 6,432us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 4,815us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Assets, Contract 11,901us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 18,280us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Fair Value 6,891us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 2,259us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Contract 10,628us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 15,202us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= kcg_FixedIncomeFuturesContractsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Commodity Futures Contracts [Member] | Derivative Instruments Not Designated as Hedging Instruments [Member] | Receivable from Brokers, Dealers and Clearing Organizations [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets, Fair Value 15,245us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 3,392us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Assets, Contract 8,894us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 10,629us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Fair Value $ 14,847us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 $ 1,773us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
Liabilities, Contract 9,105us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember		 3,806us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_BalanceSheetLocationAxis
= kcg_ReceivablesFromPayablestoBrokersDealersandClearingOrganizationsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
[1] Designated as hedging instrument.