Organization and Description of the Business	9 Months Ended
Sep. 30, 2016
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 direct-to-client and non-client exchange-based electronic market making. KCG has multiple access points to trade global equities, options, futures, fixed income, currencies and commodities via voice or automated execution. KCG was formed as a result of a strategic business combination (the “Mergers”) of Knight Capital Group, Inc.(“Knight”) and GETCO Holding Company, LLC (“GETCO”) in July 2013. As of September 30, 2016, 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. Principal trading in the Market Making segment primarily consists of direct-to-client and non-client exchange-based electronic market making. KCG 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 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 of the London Stock Exchange ("AIM"). Global Execution Services The Global Execution Services segment comprises agency execution services and trading venues, offering trading in global equities, options, futures and fixed income to institutions, banks and broker dealers. The Company generally earns commissions as an agent between principals for transactions; however, the Company may 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 electronic communication network ("ECN") that also offers trading applications; and (iv) an alternative trading system ("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 investments principally in strategic financial services-oriented 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 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 Management 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 that the disposal of a business may return a higher value to stockholders, or that the business is no longer core to its strategy, the Company may divest or exit such business. In March 2015, the Company sold KCG Hotspot, the Company's former spot institutional foreign exchange ECN, to Bats Global Markets, Inc. ("Bats"). The results of KCG Hotspot, including the gain on sale, are included in the Global Execution Services segment, up through the date of its sale. 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 details.

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 direct-to-client and non-client exchange-based electronic market making. KCG has multiple access points to trade global equities, options, futures, fixed income, currencies and commodities via voice or automated execution.

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

As of September 30, 2016, 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. Principal trading in the Market Making segment primarily consists of direct-to-client and non-client exchange-based electronic market making. KCG 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 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 of the London Stock Exchange ("AIM").

Global Execution Services

The Global Execution Services segment comprises agency execution services and trading venues, offering trading in global equities, options, futures and fixed income to institutions, banks and broker dealers. The Company generally earns commissions as an agent between principals for transactions; however, the Company may 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 electronic communication network ("ECN") that also offers trading applications; and (iv) an alternative trading system ("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 investments principally in strategic financial services-oriented 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 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

Management 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 that the disposal of a business may return a higher value to stockholders, or that the business is no longer core to its strategy, the Company may divest or exit such business.

In March 2015, the Company sold KCG Hotspot, the Company's former spot institutional foreign exchange ECN, to Bats Global Markets, Inc. ("Bats"). The results of KCG Hotspot, including the gain on sale, are included in the Global Execution Services segment, up through the date of its sale.

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 details.