Organization and Description of the Business	9 Months Ended
Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of the Business	Organization and Description of the Business On December 19, 2012, Knight Capital Group, Inc.(“Knight”), GETCO Holding Company, LLC (“GETCO”) and an affiliate of GETCO entered into an agreement and plan of merger (as amended and restated on April 15, 2013 the “Merger Agreement”) for a series of strategic business combinations (the “Mergers”). The Mergers were approved by the respective stockholders and unitholders of both companies at special meetings held on June 25, 2013, and the Mergers were completed on July 1, 2013. As a result of the Mergers, Knight and GETCO each became a wholly-owned subsidiary of KCG Holdings, Inc. (collectively with its subsidiaries, “KCG” or the “Company”). The Mergers took place in order to combine the businesses, intellectual capital and resources of the two companies to more successfully compete in the highly regulated and technologically advanced marketplace and to allow for further diversification of each company's revenues from principal and agency trading across asset classes and regions. The Mergers were treated as a purchase of Knight by GETCO for accounting and financial reporting purposes. As a result, the financial results for the third quarter of 2013 comprise three months of results of KCG, while the results for the nine months ended September 30, 2013 comprise third quarter KCG results and GETCO only results for the first half of 2013. All periods prior to 2013 comprise solely the results of GETCO. On November 12, 2013, the Company’s management concluded that, due to errors within GETCO’s Consolidated Statement of Financial Condition and related Consolidated Statement of Changes in Equity at December 31, 2012 and Consolidated Statements of Operations for the three and nine months ended September 30, 2012 and Consolidated Statement of Cash Flows for the nine months ended September 30, 2012, such documents needed to be restated to correct prior SEC filings. The financial information presented herein for December 31, 2012 and the nine months ended September 30, 2012 reflects that restatement. For more information on the restatement, see the Current Report on Form 8-K filed by the Company on November 12, 2013. KCG is a leading independent securities firm offering clients a range of services designed to address trading needs across asset classes, product types and time zones. The firm combines advanced technology with exceptional client service across market making, agency execution and trading venues. KCG has multiple access points to trade global equities, fixed income, currencies and commodities via voice or automated execution. As of September 30, 2013, the Company's operating segments comprised the following: (i) Market Making; (ii) Global Execution Services; and (iii) Corporate and Other. Market Making The Market Making segment principally consists of market making in the cash, futures and/or options markets across global equities, fixed income, commodities and foreign currencies. As a market maker, the Company commits capital for trade executions by offering to buy securities from, or sell securities to, institutions and broker-dealers. The Market Making segment primarily includes client direct and exchange-based electronic market making activities, including trade executions as an equities Designated Market Maker (“DMM”) on the New York Stock Exchange ("NYSE") and NYSE Amex Equities ("NYSE Amex"). The Company is an active participant in all major global equity and futures exchanges and also trades on substantially all domestic electronic options exchanges. As a complement to electronic market making, the Company’s cash trading business handles specialized orders and also transacts on the OTC Bulletin Board, marketplaces operated by the OTC Markets Group Inc. and the Alternative Investment Market (“AIM”) of the London Stock Exchange. Global Execution Services The Global Execution Services segment provides agency execution services and operates trading venues. This segment offers access via its electronic agency-based platforms to markets and self-directed trading in equities, options, fixed income, foreign exchange and futures. In contrast to Market Making, the Global Execution Services segment generally does not act as a principal to transactions that are executed within this segment; however, it will commit capital on behalf of clients as needed, and generally earns commissions for acting as agent between the principals to the trade. Global Execution Services includes high touch equity sales and trading (including exchange traded funds ("ETFs") aided by its network of sales employees. This segment also facilitates client orders through program, block and riskless principal trades. Additionally, the Global Execution Services segment includes the futures commission merchant ("FCM") business, which provides clients with futures execution and clearing services on major U.S. and European futures and options exchanges. Corporate and Other The Corporate and Other segment invests in strategic financial services-oriented opportunities, allocates, deploys and monitors all capital, and maintains corporate overhead expenses and all other income and expenses that are not attributable to the other segments. The Corporate and Other segment also houses functions that support the Company’s other segments such as self-clearing services, including stock lending activities. Discontinued Operations Management of the Company from time to time conducts a strategic review of its businesses and evaluates their potential value in the marketplace relative to their current and expected returns. To the extent management and the Company's Board of Directors determine a business may return a higher value to stockholders through a divestiture, or is no longer core to the Company's strategy, management may pursue a sale process. In July 2013, KCG entered into an agreement to sell Urban Financial Group, Inc. (“Urban”), the reverse mortgage origination and securitization business that it acquired as a result of the Mergers, to an investor group. The transaction is expected to be completed in the fourth quarter of 2013. As a result, the business is considered to be held for sale and its results of operations have been reported in Loss from discontinued operations, net of tax on the Consolidated Statements of Operations. Assets and Liabilities related to Urban are reported in Assets within discontinued operations and Liabilities within discontinued operations on the Consolidated Statements of Financial Condition. Completion of the sale is subject to certain customary conditions, including receipt of required regulatory approvals by the Government National Mortgage Association (“GNMA”), the U.S. Department of Housing and Urban Development ("HUD"), the Federal National Mortgage Association ("FNMA"), and states and territories in which Urban operates, and the absence of any law or order prohibiting the consummation of the sale of Urban. Discontinued operations also includes residual revenue and expenses related to Knight's former institutional fixed income sales and trading business, which was sold in June 2013, and asset management business, which Knight decided to exit prior to the closing of the Mergers. See Footnote 4 "Discontinued Operations" for further discussion.

Organization and Description of the Business

On December 19, 2012, Knight Capital Group, Inc.(“Knight”), GETCO Holding Company, LLC (“GETCO”) and an affiliate of GETCO entered into an agreement and plan of merger (as amended and restated on April 15, 2013 the “Merger Agreement”) for a series of strategic business combinations (the “Mergers”). The Mergers were approved by the respective stockholders and unitholders of both companies at special meetings held on June 25, 2013, and the Mergers were completed on July 1, 2013. As a result of the Mergers, Knight and GETCO each became a wholly-owned subsidiary of KCG Holdings, Inc. (collectively with its subsidiaries, “KCG” or the “Company”).

The Mergers took place in order to combine the businesses, intellectual capital and resources of the two companies to more successfully compete in the highly regulated and technologically advanced marketplace and to allow for further diversification of each company's revenues from principal and agency trading across asset classes and regions. The Mergers were treated as a purchase of Knight by GETCO for accounting and financial reporting purposes. As a result, the financial results for the third quarter of 2013 comprise three months of results of KCG, while the results for the nine months ended September 30, 2013 comprise third quarter KCG results and GETCO only results for the first half of 2013. All periods prior to 2013 comprise solely the results of GETCO. On November 12, 2013, the Company’s management concluded that, due to errors within GETCO’s Consolidated Statement of Financial Condition and related Consolidated Statement of Changes in Equity at December 31, 2012 and Consolidated Statements of Operations for the three and nine months ended September 30, 2012 and Consolidated Statement of Cash Flows for the nine months ended September 30, 2012, such documents needed to be restated to correct prior SEC filings. The financial information presented herein for December 31, 2012 and the nine months ended September 30, 2012 reflects that restatement. For more information on the restatement, see the Current Report on Form 8-K filed by the Company on November 12, 2013.

KCG is a leading independent securities firm offering clients a range of services designed to address trading needs across asset classes, product types and time zones. The firm combines advanced technology with exceptional client service across market making, agency execution and trading venues. KCG has multiple access points to trade global equities, fixed income, currencies and commodities via voice or automated execution.

As of September 30, 2013, the Company's operating segments comprised the following: (i) Market Making; (ii) Global Execution Services; and (iii) Corporate and Other.

Market Making

The Market Making segment principally consists of market making in the cash, futures and/or options markets across global equities, fixed income, commodities and foreign currencies. As a market maker, the Company commits capital for trade executions by offering to buy securities from, or sell securities to, institutions and broker-dealers. The Market Making segment primarily includes client direct and exchange-based electronic market making activities, including trade executions as an equities Designated Market Maker (“DMM”) on the New York Stock Exchange ("NYSE") and NYSE Amex Equities ("NYSE Amex"). The Company is an active participant in all major global equity and futures exchanges and also trades on substantially all domestic electronic options exchanges. As a complement to electronic market making, the Company’s cash trading business handles specialized orders and also transacts on the OTC Bulletin Board, marketplaces operated by the OTC Markets Group Inc. and the Alternative Investment Market (“AIM”) of the London Stock Exchange.

Global Execution Services

The Global Execution Services segment provides agency execution services and operates trading venues. This segment offers access via its electronic agency-based platforms to markets and self-directed trading in equities, options, fixed income, foreign exchange and futures. In contrast to Market Making, the Global Execution Services segment generally does not act as a principal to transactions that are executed within this segment; however, it will commit capital on behalf of clients as needed, and generally earns commissions for acting as agent between the principals to the trade. Global Execution Services includes high touch equity sales and trading (including exchange traded funds ("ETFs") aided by its network of sales employees. This segment also facilitates client orders through program, block and riskless principal trades. Additionally, the Global Execution Services segment includes the futures commission merchant ("FCM") business, which provides clients with futures execution and clearing services on major U.S. and European futures and options exchanges.

Corporate and Other

The Corporate and Other segment invests in strategic financial services-oriented opportunities, allocates, deploys and monitors all capital, and maintains corporate overhead expenses and all other income and expenses that are not attributable to the other segments. The Corporate and Other segment also houses functions that support the Company’s other segments such as self-clearing services, including stock lending activities.

Discontinued Operations

Management of the Company from time to time conducts a strategic review of its businesses and evaluates their potential value in the marketplace relative to their current and expected returns. To the extent management and the Company's Board of Directors determine a business may return a higher value to stockholders through a divestiture, or is no longer core to the Company's strategy, management may pursue a sale process.

In July 2013, KCG entered into an agreement to sell Urban Financial Group, Inc. (“Urban”), the reverse mortgage origination and securitization business that it acquired as a result of the Mergers, to an investor group. The transaction is expected to be completed in the fourth quarter of 2013. As a result, the business is considered to be held for sale and its results of operations have been reported in Loss from discontinued operations, net of tax on the Consolidated Statements of Operations. Assets and Liabilities related to Urban are reported in Assets within discontinued operations and Liabilities within discontinued operations on the Consolidated Statements of Financial Condition. Completion of the sale is subject to certain customary conditions, including receipt of required regulatory approvals by the Government National Mortgage Association (“GNMA”), the U.S. Department of Housing and Urban Development ("HUD"), the Federal National Mortgage Association ("FNMA"), and states and territories in which Urban operates, and the absence of any law or order prohibiting the consummation of the sale of Urban.

Discontinued operations also includes residual revenue and expenses related to Knight's former institutional fixed income sales and trading business, which was sold in June 2013, and asset management business, which Knight decided to exit prior to the closing of the Mergers. See Footnote 4 "Discontinued Operations" for further discussion.