Organization and Description of the Business	3 Months Ended
Mar. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of the Business	Organization and Description of the Business KCG Holdings, Inc. (collectively with its subsidiaries, "KCG" or the "Company") 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 Company combines advanced technology with specialized client service across market making, agency execution and trading venues and also engages in principal trading via exchange-based electronic market making. KCG has multiple access points to trade global equities, options, fixed income, currencies and commodities via voice or automated execution. KCG was formed as a result of a strategic business combination (the “2013 Mergers”) of Knight Capital Group, Inc.(“Knight”) and GETCO Holding Company, LLC (“GETCO”) in July 2013. Pending Merger with Virtu On April 20, 2017, the Company entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Virtu Financial, Inc., a Delaware corporation (“Virtu”) and Orchestra Merger Sub, Inc., a Delaware corporation and indirect wholly owned subsidiary of Virtu (“Merger Sub”). The Merger Agreement provides for the merger of Merger Sub with and into the Company (the “Merger”), with the Company surviving the Merger as a wholly owned subsidiary of Virtu. Pursuant to the Merger Agreement, at the effective time of the Merger (the “Effective Time”), each share of Class A common stock of the Company, par value $0.01 per share ("KCG Class A Common Stock"), issued and outstanding immediately prior to the Effective Time (other than (i) Shares owned by Virtu, Merger Sub, any affiliate of Virtu or Merger Sub, or the Company, in each case immediately prior to the Effective Time, and (ii) Shares held by stockholders who have not voted in favor of the Merger and who have properly and validly perfected their statutory rights of appraisal in respect of such Shares in accordance with Section 262 of the Delaware General Corporation Law) will be canceled and converted into the right to receive $20.00 in cash without interest (the “Merger Consideration”), subject to applicable tax withholding. In accordance with the Merger Agreement, all unvested stock-based compensation awards that are outstanding as of immediately prior to the Effective Time shall accelerate and become fully vested and exercisable effective immediately prior to, and contingent upon, the Effective Time. The respective obligations of the Company, Virtu and Merger Sub to consummate the Merger are subject to the satisfaction or waiver of certain customary conditions, including the adoption of the Merger Agreement by the Company’s stockholders, the receipt of certain required governmental approvals, the absence of any legal prohibitions, the accuracy of the representations and warranties (subject to customary materiality qualifiers) and compliance by the other party with its obligations under the Merger Agreement (subject to customary materiality qualifiers). The consummation of the Merger is not subject to a financing condition. Concurrently with the execution of the Merger Agreement, Virtu entered into a voting agreement with Jefferies LLC (“Jefferies”), a stockholder of the Company, which requires Jefferies to, among other things, vote all of its shares of KCG Class A Common Stock in favor of adoption of the Merger Agreement. The Company expects the Merger to close in the third quarter of 2017. As of March 31, 2017, 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 options markets across global equities, options, fixed income, currencies and commodities. As a market maker, the Company commits capital on a principal basis by offering to buy securities from, or sell securities to, broker dealers, banks and institutions. The Company engages in principal trading in the Market Making segment direct to clients as well as in a supplemental capacity on exchanges, electronic communications networks (“ECNs”) and alternative trading systems (“ATSs”). The Company is an active participant on 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 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 of the London Stock Exchange ("AIM"). Global Execution Services The Global Execution Services segment comprises agency-based trading and trading venues, offering execution services in global equities, options, futures and fixed income on behalf of institutions, banks and broker dealers. The Company earns commissions as an agent on behalf of clients as well as between principals to transactions; in addition, the Company will commit capital on behalf of clients as needed. Agency-based, execution-only trading in the segment is done primarily through a variety of access points including: (i) algorithmic trading and order routing in global equities and options; (ii) institutional sales traders executing program, block and riskless principal trades in global equities and exchange traded funds ("ETFs"); (iii) a fixed income ECN that also offers trading applications; and (iv) an ATS for U.S. equities. In September 2016, the Company completed the acquisition of Neonet Securities AB ("Neonet"). Neonet is an independent agency broker and execution specialist based in Stockholm, Sweden. The results of Neonet, beginning on the date of acquisition, are included in this segment. The Company does not consider this acquisition to be significant. Corporate and Other The Corporate and Other segment contains the Company's investments, principally in strategic trading-related opportunities; manages the deployment of capital across the organization; houses executive management functions; and maintains corporate overhead expenses and all other income and expenses that are not attributable to the Company's other segments. The Corporate and Other segment also contains functions that support the Company’s other segments such as self-clearing services, including stock lending activities. Sales of Businesses In March 2016, KCG completed the sale of assets related to its retail U.S. options market making business. In May 2016, KCG completed the sale of its business as an equities designated market maker ("DMM") on the New York Stock Exchange ("NYSE") to Citadel Securities LLC (“Citadel”). The results of both the retail U.S. options market making business and the DMM business are included in the Market Making segment, up through the date of the respective sales. See Footnote 3 "Assets of Businesses Held for Sale & Sales of Businesses" for further information.

Organization and Description of the Business

KCG Holdings, Inc. (collectively with its subsidiaries, "KCG" or the "Company") 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 Company combines advanced technology with specialized client service across market making, agency execution and trading venues and also engages in principal trading via exchange-based electronic market making. KCG has multiple access points to trade global equities, options, fixed income, currencies and commodities via voice or automated execution.

KCG was formed as a result of a strategic business combination (the “2013 Mergers”) of Knight Capital Group, Inc.(“Knight”) and GETCO Holding Company, LLC (“GETCO”) in July 2013.

Pending Merger with Virtu

On April 20, 2017, the Company entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Virtu Financial, Inc., a Delaware corporation (“Virtu”) and Orchestra Merger Sub, Inc., a Delaware corporation and indirect wholly owned subsidiary of Virtu (“Merger Sub”). The Merger Agreement provides for the merger of Merger Sub with and into the Company (the “Merger”), with the Company surviving the Merger as a wholly owned subsidiary of Virtu.

Pursuant to the Merger Agreement, at the effective time of the Merger (the “Effective Time”), each share of Class A common stock of the Company, par value $0.01 per share ("KCG Class A Common Stock"), issued and outstanding immediately prior to the Effective Time (other than (i) Shares owned by Virtu, Merger Sub, any affiliate of Virtu or Merger Sub, or the Company, in each case immediately prior to the Effective Time, and (ii) Shares held by stockholders who have not voted in favor of the Merger and who have properly and validly perfected their statutory rights of appraisal in respect of such Shares in accordance with Section 262 of the Delaware General Corporation Law) will be canceled and converted into the right to receive $20.00 in cash without interest (the “Merger Consideration”), subject to applicable tax withholding. In accordance with the Merger Agreement, all unvested stock-based compensation awards that are outstanding as of immediately prior to the Effective Time shall accelerate and become fully vested and exercisable effective immediately prior to, and contingent upon, the Effective Time.

The respective obligations of the Company, Virtu and Merger Sub to consummate the Merger are subject to the satisfaction or waiver of certain customary conditions, including the adoption of the Merger Agreement by the Company’s stockholders, the receipt of certain required governmental approvals, the absence of any legal prohibitions, the accuracy of the representations and warranties (subject to customary materiality qualifiers) and compliance by the other party with its obligations under the Merger Agreement (subject to customary materiality qualifiers). The consummation of the Merger is not subject to a financing condition.

Concurrently with the execution of the Merger Agreement, Virtu entered into a voting agreement with Jefferies LLC (“Jefferies”), a stockholder of the Company, which requires Jefferies to, among other things, vote all of its shares of KCG Class A Common Stock in favor of adoption of the Merger Agreement.

The Company expects the Merger to close in the third quarter of 2017.

As of March 31, 2017, 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 options markets across global equities, options, fixed income, currencies and commodities. As a market maker, the Company commits capital on a principal basis by offering to buy securities from, or sell securities to, broker dealers, banks and institutions. The Company engages in principal trading in the Market Making segment direct to clients as well as in a supplemental capacity on exchanges, electronic communications networks (“ECNs”) and alternative trading systems (“ATSs”). The Company is an active participant on 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 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 of the London Stock Exchange ("AIM").

Global Execution Services

The Global Execution Services segment comprises agency-based trading and trading venues, offering execution services in global equities, options, futures and fixed income on behalf of institutions, banks and broker dealers. The Company earns commissions as an agent on behalf of clients as well as between principals to transactions; in addition, the Company will commit capital on behalf of clients as needed. Agency-based, execution-only trading in the segment is done primarily through a variety of access points including: (i) algorithmic trading and order routing in global equities and options; (ii) institutional sales traders executing program, block and riskless principal trades in global equities and exchange traded funds ("ETFs"); (iii) a fixed income ECN that also offers trading applications; and (iv) an ATS for U.S. equities.

In September 2016, the Company completed the acquisition of Neonet Securities AB ("Neonet"). Neonet is an independent agency broker and execution specialist based in Stockholm, Sweden. The results of Neonet, beginning on the date of acquisition, are included in this segment. The Company does not consider this acquisition to be significant.

Corporate and Other

The Corporate and Other segment contains the Company's investments, principally in strategic trading-related opportunities; manages the deployment of capital across the organization; houses executive management functions; and maintains corporate overhead expenses and all other income and expenses that are not attributable to the Company's other segments. The Corporate and Other segment also contains functions that support the Company’s other segments such as self-clearing services, including stock lending activities.

Sales of Businesses

In March 2016, KCG completed the sale of assets related to its retail U.S. options market making business.

In May 2016, KCG completed the sale of its business as an equities designated market maker ("DMM") on the New York Stock Exchange ("NYSE") to Citadel Securities LLC (“Citadel”).

The results of both the retail U.S. options market making business and the DMM business are included in the Market Making segment, up through the date of the respective sales.

See Footnote 3 "Assets of Businesses Held for Sale & Sales of Businesses" for further information.