KCG Inactive 10-Q11 May 152015 Q1 Quarterly reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
Document and Entity Information	Mar. 31, 2015	7-May-15
Entity Information [Line Items]
Entity Registrant Name	KCG HOLDINGS, INC.
Entity Central Index Key	1569391
Document Type	10-Q
Document Period End Date	31-Mar-15
Amendment Flag	FALSE
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Common Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		117,941,352
Common Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		0

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues
Trading revenues, net	$208,795	$258,297
Commissions and fees	99,961	112,257
Interest, net	-23	948
Investment income and other, net	387,423	12,155
Total revenues	696,156	383,657
Expenses
Employee compensation and benefits	106,718	122,319
Execution and clearance fees	68,473	75,501
Communications and data processing	33,764	36,796
Depreciation and amortization	20,615	20,103
Payments for order flow	15,221	22,032
Professional fees	11,181	5,402
Debt interest expense	8,463	9,524
Collateralized financing interest	8,456	6,162
Occupancy and equipment rentals	7,340	8,285
Business development	1,857	1,683
Lease loss accrual, net	132	266
Writedown of capitalized debt costs	0	7,557
Other	7,808	8,643
Total expenses	290,028	324,273
Income from continuing operations before income taxes	406,128	59,384
Income tax expense	156,827	22,467
Income from continuing operations, net of tax	249,301	36,917
Loss from discontinued operations, net of tax	0	-1,253
Net income	$249,301	$35,664
Basic earnings per share from continuing operations (in dollars per share)	$2.25	$0.32
Diluted earnings per share from continuing operations (in dollars per share)	$2.19	$0.31
Basic loss per share from discontinued operations (in dollars per share)	$0	($0.01)
Diluted loss per share from discontinued operations (in dollars per share)	$0	($0.01)
Basic earnings per share (in dollars per share)	$2.25	$0.31
Diluted earnings per share (in dollars per share)	$2.19	$0.30
Shares used in computation of basic earnings (loss) per share	110,782	115,569
Shares used in computation of diluted earnings (loss) per share	113,615	117,898

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$249,301	$35,664
Other comprehensive income (loss):
Unrealized gain (loss) on available for sale securities, net of tax	188	-137
Cumulative translation adjustment, net of tax	-623	201
Comprehensive income	$248,866	$35,728

CONSOLIDATED_STATEMENTS_OF_FIN

CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Unaudited) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$990,542	$578,768
Cash and cash equivalents segregated under federal and other regulations	3,000	3,361
Funds held in escrow	330,163	0
Financial instruments owned, at fair value, including securities pledged to counterparties that had the right to deliver or repledge of $506,532 at March 31, 2015 and $536,124 at December 31, 2014:
Equities	2,366,225	2,479,910
Listed options	87,412	144,586
Debt securities	187,314	82,815
Other financial instruments	0	60
Total financial instruments owned, at fair value	2,640,951	2,707,371
Collateralized agreements:
Securities borrowed	1,685,850	1,632,062
Receivable from brokers, dealers and clearing organizations	918,506	1,188,833
Fixed assets and leasehold improvements, less accumulated depreciation and amortization	129,171	134,051
Investments	105,624	100,726
Goodwill and Intangible assets, less accumulated amortization	146,539	152,594
Deferred tax asset, net	164,298	154,759
Assets of business held for sale	0	40,484
Other assets	217,407	137,645
Total assets	7,332,051	6,830,654
Financial instruments sold, not yet purchased, at fair value:
Equities	1,950,860	2,069,342
Listed options	78,427	115,362
Debt securities	112,763	101,003
Other financial instruments	234	0
Total financial instruments sold, not yet purchased, at fair value	2,142,284	2,285,707
Collateralized financings:
Securities loaned	792,887	707,744
Financial instruments sold under agreements to repurchase	905,567	933,576
Total collateralized financings	1,698,454	1,641,320
Payable to brokers, dealers and clearing organizations	538,843	676,089
Payable to customers	12,126	22,110
Accrued compensation expense	41,831	114,559
Accrued expenses and other liabilities	161,850	136,977
Income taxes payable	148,481	0
Capital lease obligations	5,080	6,700
Liabilities of business held for sale	0	2,356
Debt	799,847	422,259
Total liabilities	5,548,796	5,308,077
Equity
Shares authorized: 1,000,000 at March 31, 2015 and December 31, 2014; Shares issued: 129,572 at March 31, 2015 and 127,508 at December 31, 2014; Shares outstanding: 118,091 at March 31, 2015 and 116,860 at December 31, 2014	1,296	1,275
Additional paid-in capital	1,391,368	1,369,298
Retained earnings	522,081	272,780
Treasury stock, at cost; 11,481 shares at March 31, 2015 and 10,649 shares at December 31, 2014	-133,188	-122,909
Accumulated other comprehensive income	1,698	2,133
Total equity	1,783,255	1,522,577
Total liabilities and equity	$7,332,051	$6,830,654

CONSOLIDATED_STATEMENTS_OF_FIN1

CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Unaudited) (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Financial instruments owned, at fair value, securities pledged	$506,532	$536,124
Common stock, shares authorized	1,000,000,000	1,000,000,000
Common stock, shares issued	129,572,000	127,508,000
Common stock, shares outstanding	118,091,000	116,860,000
Treasury stock, shares	11,481,000	10,649,000

CONSOLIDATED_STATEMENT_OF_CHAN

CONSOLIDATED STATEMENT OF CHANGES IN EQUITY (Unaudited) (USD $)	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]
In Thousands, unless otherwise specified	Total	Common Class A [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]
Balance at Dec. 31, 2014	($122,909)				($122,909)
Balance at Dec. 31, 2014	1,522,577	1,275	1,369,298	272,780		2,133
Balance (in shares) at Dec. 31, 2014		127,508
Balance (in shares) at Dec. 31, 2014	-10,649				-10,649
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased, shares					-832
KCG Class A Common Stock repurchased	-10,279				-10,279
Stock-based compensation, shares		2,064
Stock-based compensation	22,091	21	22,070
Unrealized gain on available for sale securities, net	188					188
Cumulative translation adjustment, net	-623					-623
Net income	249,301			249,301
Balance at Mar. 31, 2015	-133,188				-133,188
Balance at Mar. 31, 2015	$1,783,255	$1,296	$1,391,368	$522,081		$1,698
Balance (in shares) at Mar. 31, 2015		129,572
Balance (in shares) at Mar. 31, 2015	-11,481				-11,481

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities
Net income	$249,301	$35,664
Loss from discontinued operations, net of tax	0	-1,253
Income from continuing operations, net of tax	249,301	36,917
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by operating activities
Realized gain from sale of KCG Hotspot	-385,026	0
Depreciation and amortization	20,615	20,103
Deferred taxes	-9,539	0
Stock and unit-based compensation	15,421	17,025
Writedown and amortization of debt offering costs	934	8,766
Lease loss accrual, net	132	266
Unrealized gain on investments	-2,460	-12,738
Deferred rent	-4	-68
Operating activities from discontinued operations	0	-1,253
(Increase) decrease in operating assets
Cash and cash equivalents segregated under federal and other regulations	361	-25,754
Financial instruments owned, at fair value	66,420	132,500
Securities borrowed	-53,788	-9,663
Receivable from brokers, dealers and clearing organizations	270,328	-215,609
Other assets	-5,011	9,485
(Decrease) increase in operating liabilities
Financial instruments sold, not yet purchased, at fair value	-143,422	-83,434
Securities loaned	85,143	-8,283
Financial instruments sold under agreements to repurchase	-28,009	224,035
Payable to brokers, dealers and clearing organizations	-137,247	43,472
Payable to customers	-9,984	71,078
Accrued compensation expense	-67,071	-86,388
Accrued expenses and other liabilities	-1,133	12,141
Income taxes payable	148,481	0
Net cash provided by operating activities	14,442	132,598
Cash flows from investing activities
Cash received from sale of KCG Hotspot, net of cash provided	358,445	0
Proceeds and distributions from investments	2,013	42,365
Purchases of fixed assets and leasehold improvements	-6,824	-5,816
Capitalized software development costs	-2,668	-2,321
Purchases of investments	-2,328	-582
Net cash provided by investing activities	348,638	33,646
Cash flows from financing activities
Partial repayment of Credit Agreement	0	-185,000
Proceeds from issuance of 6.875% Senior Secured Notes, net	494,810	0
Repayment of Convertible Notes	-117,259	0
Funding of collateral account for 8.25% Senior Secured Notes	-305,000	0
Borrowings under capital lease obligations	0	4,310
Payment of debt issuance costs	-11,335	0
Principal payments on capital lease obligations	-1,620	-4,248
Cost of common stock repurchased	-10,279	-4,555
Net cash provided by (used in) financing activities	49,317	-189,493
Effect of exchange rate changes on cash and cash equivalents	-623	64
Increase (decrease) in cash and cash equivalents	411,774	-23,185
Cash and cash equivalents at beginning of period	578,768	674,281
Cash and cash equivalents at end of period	990,542	651,096
Supplemental disclosure of cash flow information:
Cash paid for interest	11,637	13,577
Cash paid for income taxes	564	793
Non-cash investing activities	$1,927	$0

Organization_and_Description_o

Organization and Description of the Business	3 Months Ended
Organization and Description of the Business	Mar. 31, 2015
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 March 31, 2015, 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, including trade executions conducted 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 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 (“AIM”) of the London Stock Exchange.
	Global Execution Services
	The Global Execution Services segment comprises agency execution services and trading venues, offering trading in global equities, futures, options, and fixed income to institutions, banks and broker dealers. The Company generally earns commissions as an agent between principals for transactions that are executed within this segment; 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; (ii) institutional sales traders executing program, block and riskless principal trades in global equities and exchange traded funds ("ETFs"); (iii) a fixed income electronic communications network ("ECN") that also offers trading applications; and (iv) an alternative trading system ("ATS") for global equities.
	Corporate and Other
	The Corporate and Other segment invests principally 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 contains functions that support the Company’s other segments.
	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 a business may return a higher value to stockholders, or is no longer core to our strategy, the Company may divest or exit such business.
	In November 2013, KCG sold Urban Financial of America, LLC, (“Urban”), the reverse mortgage origination and securitization business that was previously owned by Knight to an investor group.
	In November 2014, KCG sold certain assets and liabilities related to its Futures Commission Merchant (“FCM”) business to Wedbush Securities Inc.
	In March 2015, the Company sold KCG Hotspot, the Company's spot institutional foreign exchange ECN, to BATS Global Markets, Inc. ("BATS").
	See Footnote 3 "Sales of Businesses".

Significant_Accounting_Policie

Significant Accounting Policies

3 Months Ended

Mar. 31, 2015

Accounting Policies [Abstract]

Significant Accounting Policies

Basis of consolidation and form of presentation

The Consolidated Financial Statements, prepared in conformity with generally accepted accounting principles ("GAAP"), include the accounts of the Company and its subsidiaries. All significant intercompany transactions and balances have been eliminated.

Certain reclassifications have been made to the prior periods’ Consolidated Financial Statements in order to conform to the current period presentation. Such reclassifications are immaterial to both current and all previously issued financial statements taken as a whole and have no effect on previously reported Consolidated Net income.

Change in accounting principle

As discussed in Footnote 9 "Investments", as a result of the merger of BATS and Direct Edge Holdings LLC ("Direct Edge") in the first quarter of 2014, the Company changed its method of accounting for its investment in BATS from the cost method to the equity method.

During the first quarter of 2014, the Company recognized income of $9.6 million related to the merger of BATS and Direct Edge which is recorded within Investment income and other, net in the Consolidated Statement of Operations. The $9.6 million comprises a partial realized gain with respect to the Company's investment in Direct Edge of $16.2 million offset, in part, by the Company's share of BATS' and Direct Edge's merger related transaction costs that were charged against their respective earnings of $6.6 million.

Cash and cash equivalents

Cash and cash equivalents include money market accounts, which are payable on demand, and short-term investments with an original maturity of less than 90 days. The carrying amount of such cash equivalents approximates their fair value due to the short-term nature of these instruments. These assets would be categorized as Level 1 in the fair value hierarchy if they were required to be recorded at fair value.

Cash and cash equivalents segregated under federal and other regulations

The Company maintains custody of customer funds and is obligated by rules and regulations mandated by the U.S. Securities and Exchange Commission (“SEC”) and the Commodity Futures Trading Commission (“CFTC”) to segregate or set aside cash and/or qualified securities to satisfy these regulations, which have been promulgated to protect customer assets. The amounts recognized as Cash and cash equivalents segregated under federal and other regulations approximate fair value.

Market making, sales, trading and execution activities

Financial instruments owned and Financial instruments sold, not yet purchased, relate to market making and trading activities, include listed and other equity securities, listed equity options and fixed income securities which are recorded on a trade date basis and carried at fair value. Trading revenues, net, which comprises trading gains, net of trading losses, are also recorded on a trade date basis.

Commissions, which primarily includes commission equivalents earned on institutional client orders and related expenses are also recorded on a trade date basis. In 2014, commissions earned by the Company’s former FCM, which was sold in November 2014, were recorded net of any commissions paid to independent brokers and recognized on a half-turn basis.

The Company’s third party clearing agreements call for payment or receipt of interest income, net of transaction-related interest charged by such clearing brokers, for facilitating the settlement and financing of securities transactions. Interest income and interest expense which have been netted within Interest, net on the Consolidated Statements of Operations are as follows (in thousands):

For the three months ended March 31,

2015

2014

Interest Income

3,771

3,351

Interest Expense

(3,794

)

(2,403

)

Interest, net

(23

)

948

Dividend income relating to financial instruments owned and dividend expense relating to financial instruments sold, not yet purchased, derived primarily from the Company’s market making activities are included as a component of Trading revenues, net on the Consolidated Statements of Operations. Trading revenues, net includes dividend income and expense as follows (in thousands):

For the three months ended March 31,

2015

2014

Dividend Income

16,743

9,783

Dividend Expense

(10,253

)

(7,575

)

Payments for order flow represent payments to broker dealer clients, in the normal course of business, for directing their order flow in U.S. equities and options to the Company.

Fair value of financial instruments

The Company values its financial instruments using a hierarchy of fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs when measuring fair value.

The fair value hierarchy can be summarized as follows:

•

Level 1—Valuations based on quoted prices in active markets for identical assets or liabilities that the Company has the ability to access. Since valuations are based on quoted prices that are readily and regularly available in an active market, valuation of these products does not entail a significant degree of judgment.

•

Level 2—Valuations based on quoted prices in markets that are not active or for which all significant inputs are observable, either directly or indirectly.

•

Level 3—Valuations based on inputs that are unobservable and significant to the overall fair value measurement.

Changes in fair value are recognized in earnings each period for financial instruments that are carried at fair value. See Footnote 5 “Fair Value of Financial Instruments” for a description of valuation methodologies applied to the classes of financial instruments at fair value.

Collateralized agreements and financings

Collateralized agreements consist of securities borrowed and collateralized financings include securities loaned and financial instruments sold under agreements to repurchase.

•

Securities borrowed and securities loaned transactions are recorded at the amount of cash collateral advanced or received. Securities borrowed transactions facilitate the securities settlement process and require the Company to deposit cash or other collateral with the lender. Securities loaned transactions help finance the Company’s securities inventory whereby the Company lends stock to counterparties in exchange for the receipt of cash or other collateral from the borrower. In these transactions, the Company receives or lends cash or other collateral in an amount generally in excess of the market value of the applicable securities borrowed or loaned. The Company monitors the market value of securities borrowed or loaned on a daily basis, and obtains additional collateral or refunds excess collateral as necessary.

•

Financial instruments sold under agreements to repurchase are used to finance inventories of securities and other financial instruments and are recorded at their contractual amount. The Company has entered into bilateral and tri-party term and overnight repurchase agreements which bear interest at negotiated rates. The Company receives cash and makes delivery of financial instruments to a custodian who monitors the market value of these instruments on a daily basis. The market value of the instruments delivered must be equal to or in excess of the principal amount loaned under the repurchase agreements plus the agreed upon margin requirement. The custodian may request additional collateral, if appropriate.

The Company’s securities borrowed, securities loaned and financial instruments sold under agreements to repurchase are recorded at amounts that approximate fair value. These items are recorded based upon their contractual terms and are not materially sensitive to shifts in interest rates because they are short-term in nature and are substantially collateralized pursuant to the terms of the underlying agreements. These items would be categorized as Level 2 in the fair value hierarchy if they were required to be recorded at fair value.

Investments

Investments primarily comprise strategic noncontrolling equity ownership interests in financial services-related businesses and are held by the Company's non-broker dealer subsidiaries. These strategic investments are accounted for under the equity method, at cost or at fair value. The equity method of accounting is used when the Company has significant influence. Strategic investments are held at cost, less impairment if any, when the investment does not have a readily determined fair value, and the Company is not considered to exert significant influence on operating and financial policies of the investee. Strategic investments that are publicly traded are held at fair value and classified as available for sale securities on the Consolidated Statements of Financial Condition.

Strategic investments are reviewed on an ongoing basis to ensure that the carrying values of the investments have not been impaired. If the Company determines that an impairment loss on a strategic investment has occurred due to a decline in fair value or other market conditions, the investment is written down to its estimated fair value.

The Company maintains a non-qualified deferred compensation plan for certain employees and directors. This plan provides a return to the participants based upon the performance of various investments. In order to hedge its liability under this plan, the Company generally acquires the underlying investments and holds such investments until the deferred compensation liabilities are satisfied. Changes in value of such investments are recorded in Investment income and other, net, with a corresponding charge or credit to Employee compensation and benefits on the Consolidated Statements of Operations. Deferred compensation investments primarily consist of mutual funds, which are accounted for at fair value.

Goodwill and intangible assets

The Company tests goodwill and intangible assets with an indefinite useful life for impairment annually or when an event occurs or circumstances change that signifies that the carrying amounts may not be recoverable. The Company capitalizes certain costs associated with the acquisition or development of internal-use software and amortizes the software over its estimated useful life of three years, commencing at the time the software is placed in service. The Company amortizes intangible assets with a finite life on a straight line basis over their estimated useful lives and tests for recoverability whenever events indicate that the carrying amounts may not be recoverable.

Payable to customers

Payable to customers primarily relate to amounts due on cash and margin transactions. Due to their short-term nature, such amounts approximate fair value.

Treasury stock

The Company records its purchases of treasury stock at cost as a separate component of stockholders’ equity. The Company may obtain treasury stock through purchases in the open market or through privately negotiated transactions. Certain treasury stock repurchases represent shares of KCG Class A Common Stock repurchased in satisfaction of tax withholding obligations upon vesting of restricted awards. The Company may re-issue treasury stock, at average cost, for the acquisition of new businesses and in certain other circumstances.

Foreign currency translation and foreign currency forward contracts

The Company's foreign subsidiaries generally use the U.S. dollar as their functional currency. Effective January 1, 2014, one of the Company's U.K. subsidiaries changed its functional currency from British pounds to U.S. dollars. The Company has a subsidiary in India that utilizes the Indian Rupee as its functional currency.

Assets and liabilities of the Indian subsidiary are translated at exchange rates at the end of a period. Revenues and expenses are translated at average exchange rates during the period. Gains and losses resulting from translating foreign currency financial statements into U.S. dollars are included in Accumulated other comprehensive income on the Consolidated Statements of Financial Condition and Cumulative translation adjustment, net of tax on the Consolidated Statements of Comprehensive Income.

Gains or losses resulting from foreign currency transactions are included in Investment income and other, net on the Company’s Consolidated Statements of Operations. For the three months ended March 31, 2015 and 2014, the Company recorded losses of $0.2 million and $0.8 million, respectively on foreign currency transactions.

The Company seeks to reduce the impact of fluctuations in foreign exchange rates on its net investment in certain non-U.S. operations through the use of foreign currency forward contracts. For foreign currency forward contracts designated as hedges, the Company assesses its risk management objectives and strategy, including identification of the hedging instrument, the hedged item and the risk exposure and how effectiveness is to be assessed prospectively and retrospectively. The effectiveness of the hedge is assessed based on the overall changes in the fair value of the forward contracts.

Stock and unit based compensation

Stock and unit based compensation is measured based on the grant date fair value of the awards. These costs are amortized over the requisite service period, which is typically the vesting period. Expected forfeitures are considered in determining stock-based employee compensation expense.

The Company applies a non-substantive vesting period approach for stock-based awards related to KCG Class A Common Stock whereby the expense is accelerated for those employees and directors that receive options, stock appreciation rights ("SARs") and restricted stock units ("RSUs") and are eligible to retire prior to the vesting of such awards and in certain other circumstances.

Soft dollar expense

Under a commission management program, the Company allows institutional clients to allocate a portion of their gross commissions to pay for research and other services provided by third parties. As the Company acts as an agent in these transactions, it records such expenses on a net basis within Commissions and fees on the Consolidated Statements of Operations.

Depreciation, amortization and occupancy

Fixed assets are depreciated on a straight-line basis over their estimated useful lives of three to seven years. Leasehold improvements are being amortized on a straight-line basis over the shorter of the term of the related office lease or the expected useful life of the assets. The Company reviews fixed assets and leasehold improvements for impairment whenever events or changes in circumstances indicate that the related carrying amounts may not be recoverable.

The Company recognizes rent expense under operating leases with fixed rent escalations, lease incentives and free rent periods on a straight-line basis over the lease term beginning on the date the Company takes possession of or controls the use of the space, including during free rent periods.

Lease loss accrual

The Company’s policy is to identify excess real estate capacity and where applicable, accrue for related future costs, net of projected sub-lease income upon the date the Company ceases to use the excess real estate. Such accrual is adjusted to the extent the actual terms of sub-leased property differ from the assumptions used in the calculation of the accrual.

Income taxes

The Company is a corporation subject to U.S. corporate income tax as well as non-U.S. income taxes in the jurisdictions in which it operates. The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the financial reporting and tax bases of assets and liabilities and measures them using the enacted tax rates and laws that will be in effect when such differences are expected to reverse. The Company evaluates the recoverability of future tax deductions by assessing the adequacy of future expected taxable income from all sources, including reversal of temporary differences and forecasted operating earnings.

Variable interest entities

A variable interest entity (“VIE”) is an entity that lacks one or more of the following characteristics (i) the total equity investment at risk is sufficient to enable the entity to finance its activities independently and (ii) the equity holders have the power to direct the activities of the entity that most significantly impact its economic performance, the obligation to absorb the losses of the entity and the right to receive the residual returns of the entity.

The Company will be considered to have a controlling financial interest and will consolidate a VIE if it has both (i) the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance and (ii) the obligation to absorb losses of the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE.

Effective January 2015, the Company owns 50% of the voting shares and 50% of the equity of a joint venture (“JV”) which maintains microwave communication networks in the U.S. and Europe. The Company and its JV partner each use the microwave networks in connection with their respective trading activities, and the JV sells excess bandwidth that is not utilized by the JV members to third parties. The Company pays the JV for the communication services that it uses, and such amounts are recorded within Communications and data processing on the Consolidated Statements of Operations.

The Company does not have the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance; therefore it does not have a controlling financial interest in the JV and does not consolidate the JV. The Company records its interest in the JV under the equity method of accounting and records its investment in the JV within Investments and its amounts payable for communication services within Accrued expenses and other liabilities on the Consolidated Statements of Financial Condition. The Company records its pro-rata share of the JV’s earnings or losses within Investment income and other, net on the Consolidated Statements of Operations.

The Company’s exposure to the obligations of this VIE is generally limited to its interests in the JV, which is the carrying value of the equity investment in the JV.

The following table presents the Company’s nonconsolidated VIE at March 31, 2015 (in thousands):

Carrying Amount

Maximum Exposure to loss

Asset

Liability

VIE's assets

Equity investment

2,906

727

2,906

8,876

Use of estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates.

Recently adopted accounting guidance

In April 2014, the Financial Accounting Standards Board (“FASB”) issued an Accounting Standard Update (“ASU”) that amends the requirements for reporting discontinued operations. Under the new guidance, discontinued operations reporting are limited to disposal transactions that represent strategic shifts having a major effect on operations and financial results. The amended guidance also enhances disclosures and requires assets and liabilities of a discontinued operation to be classified as such for all periods presented in the financial statements. The updated guidance is effective prospectively to all disposals occurring for interim and annual reporting periods after December 15, 2014, with early adoption permitted. The Company early adopted this ASU in 2014, which resulted in additional disclosures within the Company's Consolidated Financial Statements.

In June 2014, the FASB issued an ASU that amends the accounting and disclosure guidance on repurchase agreements. The amended guidance requires entities to account for repurchase-to-maturity transactions as secured borrowings. Additional disclosures will be required for the nature of collateral pledged in repurchase agreements and similar transactions accounted for as secured borrowings. The accounting changes and additional disclosures about certain transferred financial assets accounted for as sales were effective for reporting periods beginning after December 15, 2014. The additional disclosures for securities financing transactions are required for annual reporting periods beginning after December 15, 2014 and for interim reporting periods beginning after March 15, 2015. Other than additional disclosure requirements, the adoption of this ASU did not have an impact on the Company’s Consolidated Financial Statements.

Recent accounting guidance to be adopted in future periods

In May 2014, the FASB issued an ASU that updates the principles for recognizing revenue. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2016. Early adoption is not permitted. The Company is evaluating the impact of this ASU on its Consolidated Financial Statements.

In June 2014, the FASB issued an ASU to resolve diverse accounting treatment for share based awards in which the terms of the award are related to a performance target that affects vesting. The ASU requires an entity to treat a performance target that could be achieved after the requisite service period as a performance condition. Additionally, compensation cost should be recognized in the period in which it becomes probable that the performance target will be achieved, and should represent the compensation cost attributable to the period(s) for which the requisite service has already been rendered; if the performance target becomes probable of being achieved before the end of the requisite service period, then the remaining unrecognized compensation cost should be recognized prospectively over the remaining requisite service period. The guidance is effective for reporting periods beginning after December 15, 2015 and may be applied prospectively or retrospectively. The Company does not expect adoption of this ASU to have an impact on the Company’s Consolidated Financial Statements.

In August 2014, the FASB issued an ASU that requires an entity’s management to evaluate whether there are conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. The guidance is effective for reporting periods beginning after December 15, 2016. Other than additional disclosure requirements, the adoption of this ASU is not expected to have an impact on the Company’s Consolidated Financial Statements.

In February 2015, the FASB issued an ASU which requires entities to evaluate whether they should consolidate certain legal entities. The ASU simplifies consolidation accounting by reducing the number of consolidation models that an entity may apply. The guidance is effective for the Company beginning after December 15, 2015 and early adoption is permitted. The Company is evaluating the impact of this ASU on its Consolidated Financial Statements.

In April 2015, the FASB issued an ASU regarding simplification of the presentation of debt issuance costs. The ASU requires that debt issuance costs related to a recognized debt liability be presented in the Consolidated Statement of Financial Condition as a direct deduction from the carrying amount of that debt liability. The guidance is effective for the Company retrospectively for fiscal years beginning after December 15, 2015 and early adoption is permitted. The Company is evaluating the impact of this ASU on its Consolidated Financial Statements.

Sales_of_Businesses

Sales of Businesses	3 Months Ended
	Mar. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Sales of Businesses	Sales of Businesses
	In July 2013, the Company entered into an agreement to sell to an investor group Urban, the reverse mortgage origination and securitization business that was previously owned by Knight. The transaction was completed in November 2013, and, as a result, the revenues and expenses of Urban's operations and costs of the related sale have been included in Loss from discontinued operations, net of tax within the Consolidated Statements of Operations for the three months ended March 31, 2014.
	The revenues and results of operations of discontinued operations are summarized as follows (in thousands):


		For the three months ended March 31, 2014
	Additional Loss on Sale	$	(1,312	)

	Expenses:
	Compensation	$	46

	Other expenses	662

	Total Expenses	708

	Loss from discontinued operations	(2,020		)
	Income tax benefit	767

	Loss from discontinued operations, net of tax	$	(1,253	)
	In September 2014, KCG entered into an agreement to sell certain assets and liabilities related to its FCM business to Wedbush Securities Inc. The transaction closed on November 30, 2014 and as such, there are no assets or liabilities related to the FCM business on the December 31, 2014 Consolidated Statement of Financial Condition. As a result of KCG’ s decision to early adopt the aforementioned ASU that amended the requirements for reporting discontinued operations, the FCM is not considered a discontinued operation, and therefore the results of the FCM’s operations for the three months ended March 31, 2014 are included in the Global Execution Services segment and in Continuing Operations on the Consolidated Statements of Operations.
	In October 2014, the Company announced that it began to explore strategic options for KCG Hotspot. KCG Hotspot was a single disposal group that was considered to be held-for-sale as of December 31, 2014 and, as a result, certain assets and liabilities related to KCG Hotspot were included in Assets of business held for sale and Liabilities of business held for sale on the Consolidated Statement of Financial Condition as of December 31, 2014. The Company determined that the sale of KCG Hotspot did not represent a strategic shift that will have a major effect on its operations and financial results, and therefore KCG Hotspot did not meet the requirements to be treated as a discontinued operation under the recently adopted ASU relating to discontinued operations. As such, the results of KCG Hotspot's operations are included in the Global Execution Services segment and in Continuing Operations on the Consolidated Statements of Operations for all applicable periods presented, including the first quarter of 2015, through the sale date of March 13, 2015.
	In March 2015, the Company completed the sale of KCG Hotspot to BATS. The Company and BATS have agreed to share certain tax benefits, which could result in future payments to the Company of up to approximately $70.0 million in the three-year period following the close. The additional potential payments were recorded in Other assets on the Consolidated Statements of Financial Condition as of March 31, 2015, at their estimated fair value of $62.1 million. A portion of these additional payments is contingent on KCG Hotspot achieving various levels of trading volumes through June 2015 and BATS generating sufficient taxable net income to receive the tax benefits. The Company recorded a gain on the sale of $385.0 million, which is recorded as Investment income and other, net on the Consolidated Statements of Operations for the three months ended March 31, 2015. The net gain on the sale of Hotspot was $373.8 million including direct costs associated with the sale which comprised professional fees of $6.7 million and compensation of $4.5 million, which are recorded in Professional fees and Employee compensation and benefits, respectively, on the Consolidated Statements of Operations for the three months ended March 31, 2015.
	The Company has elected the fair value option related to the $62.1 million receivable from BATS. It considers the $62.1 million receivable to be a Level 2 asset in the fair value hierarchy as the fair value is derived from observable inputs such as projected cash flows and market discount rates.
	The KCG Hotspot assets and liabilities held for sale as of December 31, 2014 are summarized as follows (in thousands):


			December 31,
			2014
	Assets:
	Fixed assets, less accumulated depreciation		$	391

	Intangible assets, net accumulated amortization		34,696

	Other assets		5,397

	Total assets of business held for sale		$	40,484


	Liabilities:
	Accrued compensation expense		$	2,298

	Accrued expenses and other liabilities		58

	Total liabilities of business held for sale		$	2,356

Assets_Segregated_or_Held_in_S

Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations	3 Months Ended
	Mar. 31, 2015
Regulated Operations [Abstract]
Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations	Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations
	Cash and securities segregated under U.S. federal and other regulations relate to the Company’s regulated businesses and consist of the following (in thousands):


		March 31,		31-Dec-14
		2015
	Cash and cash equivalents segregated under federal or other regulations	$	3,000	$	3,361

	Total assets segregated or held in separate accounts under federal or other regulations	$	3,000	$	3,361

Fair_Value

Fair Value	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value	Fair Value
	The Company’s financial instruments recorded at fair value have been categorized based upon a fair value hierarchy in accordance with accounting guidance, as described in Footnote 2 “Significant Accounting Policies.” The following fair value hierarchy table presents information about the Company’s financial assets and liabilities measured at fair value (in thousands):


		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	March 31, 2015	Level 1			Level 2			Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,366,225		$	—		$	—		$	2,366,225

	Listed options	87,412			—			—			87,412

	U.S. government and Non-U.S. government obligations	118,563			—			—			118,563

	Corporate debt (2)	68,751			—			—			68,751

	Total Financial instruments owned, at fair value	2,640,951			—			—			2,640,951

	Investment in CME Group (3)	4,738			—			—			4,738

	Other investments(3)	—			1,569			—			1,569

	Total assets held at fair value	$	2,645,689		$	1,569		$	—		$	2,647,258

	Liabilities

	Financial instruments sold, not yet purchased, at fair value:
	Equities (1)	$	1,950,860		$	—		$	—		$	1,950,860

	Listed options	78,427			—			—			78,427

	U.S. government obligations	42,108			—			—			42,108

	Corporate debt (2)	70,655			—			—			70,655

	Foreign currency forward contracts	—			234			—			234

	Total liabilities held at fair value	$	2,142,050		$	234		$	—		$	2,142,284


	(1)	Equities of $696.3 million have been netted by their respective long and short positions by CUSIP number.

	(2)	Corporate debt securities of $16,000 have been netted by their respective long and short positions by CUSIP number

	(3)	Investment in CME Group and Other investments, which primarily consist of deferred compensation investments, are included within Investments on the Consolidated Statements of Financial Condition.



		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	December 31, 2014	Level 1			Level 2			Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,479,910		$	—		$	—		$	2,479,910

	Listed options	144,586			—			—			144,586

	U.S. government and Non-U.S. government obligations	22,983			—			—			22,983

	Corporate debt (2)	59,832			—			—			59,832

	Foreign currency forward contracts	—			60			—			60

Total Financial instruments owned, at fair value	2,707,311			60			—			2,707,371

Investment in CME Group (3)	4,435			—			—			4,435

Other investments (3)	—			1,014			—			1,014

Total assets held at fair value	$	2,711,746		$	1,074		$	—		$	2,712,820

Liabilities
Financial instruments sold, not yet purchased, at fair value:
Equities (1)	$	2,069,342		$	—		$	—		$	2,069,342

Listed options	115,362			—			—			115,362

U.S. government obligations	18,953			—			—			18,953

Corporate debt (2)	82,050			—			—			82,050

Total liabilities held at fair value	$	2,285,707		$	—		$	—		$	2,285,707

(1) Equities of $743.1 million have been netted by their respective long and short positions by CUSIP number.
(2) Corporate debt instruments of $0.3 million have been netted by their respective long and short positions by CUSIP number.
(3) Investment in CME Group and Other investments, which primarily consist of deferred compensation investments, are included within Investments on the Consolidated Statements of Financial Condition.
The Company’s equities, listed options, U.S. government and non-U.S. government obligations, corporate debt and strategic investments that are publicly traded are generally classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices or broker or dealer quotations with reasonable levels of price transparency.
The types of instruments that trade in markets that are not considered to be active, but are valued based on observable inputs such as quoted market prices or alternative pricing sources with reasonable levels of price transparency are generally classified within Level 2 of the fair value hierarchy.
As of March 31, 2015 and December 31, 2014 the Company had no financial instruments classified within Level 3 of the fair value hierarchy.
The Company’s assets measured at fair value on a nonrecurring basis solely relate to goodwill and intangible assets arising from various acquisitions which would be classified as Level 3 within the fair value hierarchy. See Footnote 10 “Goodwill and Intangible Assets” for additional information.
There were no transfers of assets or liabilities held at fair value between levels of the fair value hierarchy for any periods presented.
The Company’s foreign currency forward contracts, and other investments are classified within Level 2 of the fair value hierarchy.
The following is a description of the valuation basis, techniques and significant inputs used by the Company in valuing its Level 2 assets and liabilities:
Foreign currency forward contracts
At March 31, 2015 and December 31, 2014, the Company had a foreign currency forward contract with a notional value of 700.0 million Indian Rupees ($10.9 million U.S. dollars). This forward contract is used to hedge the Company’s investment in its Indian subsidiary. The Indian Rupee foreign currency forward contract does not qualify as a net investment hedge and any gains and losses are recorded in Investment income and other, net on the Consolidated Statements of Operations.
The fair value of this forward contract was determined based upon spot foreign exchange rates, LIBOR interest rates and dealer quotations.
Other investments
Other investments primarily consist of deferred compensation investments which comprise investments in liquid mutual funds that the Company acquires to hedge its obligations to employees and directors under certain non-qualified deferred compensation arrangements. These mutual fund investments can generally be redeemed at any time and are valued based upon quoted market prices.

Derivative_Financial_Instrumen

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	Derivative Financial Instruments
	The Company enters into derivative transactions, primarily with respect to making markets in listed domestic options. In addition, the Company enters into derivatives to manage foreign currency exposure. Cash flows associated with such derivative activities are included in cash flows from operating activities on the Consolidated Statements of Cash Flows, when applicable.
	During the normal course of business, the Company enters into futures contracts. These financial instruments are subject to varying degrees of risks whereby the fair value of the securities underlying the financial instruments, may be in excess of, or less than, the contract amount. The Company is obligated to post collateral against certain futures contracts.
	The amounts and positions included in the tables below for futures contracts are classified as Level 1 while swaps and forward contracts are classified as Level 2 in the fair value hierarchy.
	The following tables summarize the fair value and number of derivative instruments held at March 31, 2015 and December 31, 2014 and the gains and losses included in the Consolidated Statements of Operations for the periods then ended. These instruments include those classified as Financial Instruments, owned at fair value, Financial instruments sold, not yet purchased at fair value, as well as futures contracts which are reported within Receivable from or Payable to brokers, dealers and clearing organizations in the Consolidated Statements of Financial Condition (fair value and gain (loss) in thousands):


				31-Mar-15
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	722			9,673			$	1,279		10,754

	Forward contracts	Financial instruments owned at fair value		—				—			234			1

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		2,250				2,208			894			3,429

	Swap contracts	Receivable from brokers, dealers and clearing organizations		46				1			—			—

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		87,412				261,752			78,427			256,523

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		4,409				6,896			8,061			11,399

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		21,535				12,459			22,332			12,050

	Total			$	116,374			292,989			$	111,227		294,156



				31-Dec-14
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	1,212			8,108			$	651		9,090

	Forward contracts	Financial instruments owned at fair value		60				1			—			—

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		1,790				2,590			2,047			3,085

	Swap contracts	Receivable from/Payable to brokers, dealers and clearing organizations		98				1			13			1

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		144,586				426,747			115,362			437,383

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		6,432				11,901			6,891			10,628

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		15,246				8,894			14,847			9,105

	Total			$	169,424			458,242			$	139,811		469,292



					Gain (Loss) Recognized
			Financial Statements		For the three months ended March 31,
			Location		2015				2014
	Derivative instruments not designated as hedging instruments:
	Foreign currency
	Futures contracts		Trading revenues, net		$	1,008			$	2,842

Forward contracts		Investment income and other, net		(294		)		432

Equity
Futures contracts		Trading revenues, net		2,377				8,737

Swap contracts		Trading revenues, net		329				1,282

Listed options (1)		Trading revenues, net		(13,550		)		(73,876		)
Fixed income
Futures contracts		Trading revenues, net		8,544				8,829

Commodity
Futures contracts		Trading revenues, net		15,435				13,570

				$	13,849			$	(38,184	)

Derivative instruments designated as hedging instruments:
Foreign exchange - forward contract		Accumulated other comprehensive income		$	—			$	168


(1)	Realized gains and losses on listed equity options relate to the Company’s market making activities in such options. Such market making activities also comprise trading in the underlying equity securities with gains and losses on such securities generally offsetting the gains and losses reported in this table. Gains and losses on such equity securities are also included in Trading revenues, net on the Company’s Consolidated Statements of Operations.
The Company has entered into and may continue to enter into International Swaps and Derivative Association, Inc (“ISDA”) master netting agreements with counterparties. Master agreements provide protection in bankruptcy in certain circumstances and, where legally enforceable, enable receivables and payables with the same counterparty to be settled or otherwise eliminated by applying amounts due against all or a portion of an amount due from the counterparty or a third party. The Company may also enter into bilateral trading agreements and other customer agreements that provide for the netting of receivables and payables with a given counterparty as a single net obligation.
Under the ISDA master netting agreements, the Company typically also executes credit support annexes, which provide for collateral, either in the form of cash or securities, to be posted or paid by a counterparty based on the fair value of the derivative receivable or payable based on the rates and parameters established in the credit support annex. In the event of counterparty’s default, provisions of the ISDA master agreement permit acceleration and termination of all outstanding transactions covered by the agreement such that a single amount is owed by, or to, the non-defaulting party. Any collateral posted can be applied to the net obligations, with any excess returned and the collateralized party has a right to liquidate the collateral. Any residual claim after netting is treated along with other unsecured claims in bankruptcy court.
The Company is also a party to clearing agreements with various central clearing parties. Under these arrangements, the central clearing counterparty facilitates settlement between counterparties based on the net payable owed or receivable due and, with respect to daily settlement, cash is generally only required to be deposited to the extent of the net amount. In the event of default, a net termination amount is determined based on the market values of all outstanding positions and the clearing organization or clearing member provides for the liquidation and settlement of the net termination amount among all counterparties to the open derivative contracts.
The table below provides information regarding (1) the extent to which, under enforceable master netting arrangements, such balances are presented net in the Consolidated Statements of Financial Condition as appropriate under GAAP and (2) the extent to which other rights of offset associated with these arrangements exist and could have had an effect on our financial position (in thousands):


	March 31, 2015	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition (3)				Net Amounts Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Available Collateral(1)		Counterparty Netting(2)

	Assets
	Listed options	$	87,412			$	—			$	87,412		$	—	$	—	$	87,412

	Swaps	46				—				46			—		—		46

	Futures	28,916				28,916				—			—		—		—

	Total Assets	$	116,374			$	28,916			$	87,458		$	—	$	—	$	87,458

	Liabilities
	Listed options	$	78,427			$	—			$	78,427		$	—	$	14,841	$	63,586

	Foreign currency forward contracts	234				—				234			—		—		234

	Futures	32,566				32,566				—			—		—		—

	Total Liabilities	$	111,227			$	32,566			$	78,661		$	—	$	14,841	$	63,820

(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
(2) Under master netting agreements with its counterparties, the Company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
(3) The full amount of the Liabilities related to Futures of $32.6 million has been netted against the Asset related to Futures of $28.9 million as the Company maintains margin in excess of the net liability and such margin is utilized to offset any excess liability.




	31-Dec-14	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Available Collateral(1)		Counterparty Netting(2)

	Assets
	Listed options	$	144,586			$	—			$	144,586		$	—	$	—	$	144,586

	Foreign currency forward contracts	60				—				60			—		—		60

	Swaps	98				13				85			—		—		85

	Futures	24,680				24,436				244			—		—		244

	Total Assets	$	169,424			$	24,449			$	144,975		$	—	$	—	$	144,975

	Liabilities
	Listed options	$	115,362			$	—			$	115,362		$	—	$	17,359	$	98,003

	Futures	24,436				24,436				—			—		—		—

	Swaps	13				13				—			—		—		—

	Total Liabilities	$	139,811			$	24,449			$	115,362		$	—	$	17,359	$	98,003

(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
(2) Under master netting agreements with its counterparties, the company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk
As a market maker in global equities, fixed income, futures, options, commodities and currencies, the majority of the Company’s securities transactions are conducted as principal or riskless principal with broker dealers and institutional counterparties primarily located in the United States. The Company self-clears substantially all of its U.S. equity and option securities transactions. The Company clears a portion of its securities transactions through third party clearing brokers. Foreign transactions are settled pursuant to global custody and clearing agreements with major U.S. banks. Substantially all of the Company’s credit exposures are concentrated with its clearing brokers, broker dealer and institutional counterparties. The Company’s policy is to monitor the credit standing of counterparties with which it conducts business.
In the normal course of its operations, the Company enters into contracts that contain a variety of representations and warranties which provide general indemnifications. The Company’s maximum exposure under these arrangements is unknown, as this would involve future claims that may be made against the Company that have not yet occurred. However, based on experience, the Company believes the risk of significant loss is minimal.
Financial instruments sold, not yet purchased, at fair value represent obligations to purchase such securities (or underlying securities) at a future date. The Company may incur a loss if the market value of the securities subsequently increases.
The Company currently has no loans outstanding to any former or current executive officer or director.

Collateralized_Transactions

Collateralized Transactions	3 Months Ended
	Mar. 31, 2015
Collateralized Agreements [Abstract]
Collateralized Transactions	Collateralized Transactions
	The Company receives financial instruments as collateral in connection with securities borrowed and financial instruments purchased under agreements to resell. Such financial instruments generally consist of equities, convertible securities and obligations of the U.S. government, but may also include obligations of federal agencies, foreign governments and corporations. In most cases the Company is permitted to deliver or repledge these financial instruments in connection with securities lending, other secured financings or for meeting settlement obligations.
	The table below presents financial instruments at fair value received as collateral and included within Securities borrowed or Receivable from brokers, dealers and clearing organizations on the Consolidated Statements of Financial Condition that were permitted to be delivered or repledged and that were delivered or repledged by the Company as well as the fair value of financial instruments which could be further repledged by the receiving party (in thousands):


		March 31,				December 31,
		2015				2014
	Collateral permitted to be delivered or repledged	$	1,614,177			$	1,586,700

	Collateral that was delivered or repledged	1,508,355				1,485,267

	Collateral permitted to be further repledged by the receiving counterparty	239,757				147,696

	In order to finance securities positions, the Company also pledges financial instruments that it owns to counterparties who, in turn, are permitted to deliver or repledge them. Under these transactions, the Company pledges certain financial instruments owned to collateralize repurchase agreements and other secured financings. Repurchase agreements and other secured financings are short-term and mature within one year. Financial instruments owned and pledged to counterparties that do not have the right to sell or repledge such financial instruments consist of equity securities.
	The table below presents information about assets pledged by the Company (in thousands):


		March 31,				December 31,
		2015				2014
	Financial instruments owned, at fair value, pledged to counterparties that have the right to deliver or repledge	$	506,532			$	536,124

	Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	951,587				979,652

	The Company may enter into master netting agreements and collateral arrangements with counterparties in order to manage its exposure to credit risk associated with securities financing transactions. Such transactions are generally executed under standard industry agreements, including, but not limited to, master securities lending agreements (securities lending transactions) and master repurchase agreements (repurchase transactions). Master agreements provide protection in bankruptcy in certain circumstances and, where legally enforceable, enable receivables and payables with the same counterparty to be settled or otherwise eliminated by applying amounts due against all or a portion of an amount due from the counterparty or a third party. The Company may also enter into bilateral trading agreements and other customer agreements that provide for the netting of receivables and payables with a given counterparty as a single net obligation.
	In the event of counterparty’s default, provisions of the master agreement permit acceleration and termination of all outstanding transactions covered by the agreement such that a single amount is owed by, or to, the non-defaulting party. Any collateral posted can be applied to the net obligations, with any excess returned and the collateralized party has a right to liquidate the collateral. Any residual claim after netting is treated along with other unsecured claims in bankruptcy court.
	The Company is also a party to clearing agreements with various central clearing parties. Under these arrangements, the central clearing counterparty facilitates settlement between counterparties based on the net payable owed or receivable due and, with respect to daily settlement, cash is generally only required to be deposited to the extent of the net amount. In the event of default, a net termination amount is determined based on the market values of all outstanding positions and the clearing organization or clearing member provides for the liquidation and settlement of the net termination amount among all counterparties to the open repurchase and/or securities lending transactions.
	The gross amounts of assets and liabilities subject to netting and gross amounts offset in the Consolidated Statements of Financial Condition were as follows (in thousands):


		March 31, 2015	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts Presented in the Statements of Financial Condition				Gross Amounts Not Offset in the Statement of Financial Condition							Net Amount
															Available Collateral(1)				Counterparty Netting(2)

		Assets
		Securities borrowed	$	1,685,850			$	—			$	1,685,850			$	1,612,086			$	13,355		$	60,409

		Receivable from brokers, dealers and clearing organizations (3)	133,935				—				133,935				133,829				—			106

		Total Assets	$	1,819,785			$	—			$	1,819,785			$	1,745,915			$	13,355		$	60,515

		Liabilities
		Securities loaned	$	792,887			$	—			$	792,887			$	760,775			$	13,355		$	18,757

		Financial instruments sold under agreements to repurchase	905,567				—				905,567				905,508				—			59

		Total Liabilities	$	1,698,454			$	—			$	1,698,454			$	1,666,283			$	13,355		$	18,816

	(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
	(2) Under master netting agreements with its counterparties, the Company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
	(3) Represents financial instruments repurchased under agreement to resell.


		31-Dec-14	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts Presented in the Statements of Financial Condition				Gross Amounts Not Offset in the Statement of Financial Condition							Net Amount
															Available Collateral(1)				Counterparty Netting(2)

		Assets
		Securities borrowed	$	1,632,062			$	—			$	1,632,062			$	1,570,194			$	15,782		$	46,086

		Receivable from brokers, dealers and clearing organizations (3)	21,545				—				21,545				21,425				—			120

		Total Assets	$	1,653,607			$	—			$	1,653,607			$	1,591,619			$	15,782		$	46,206

		Liabilities
		Securities loaned	$	707,744			$	—			$	707,744			$	682,389			$	15,782		$	9,573

	Financial instruments sold under agreements to repurchase	933,576				—				933,576				933,560				—			16

	Total Liabilities	$	1,641,320			$	—			$	1,641,320			$	1,615,949			$	15,782		$	9,589

(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
(2) Under master netting agreements with its counterparties, the company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
(3) Represents financial instruments purchased under agreement to resell.
See Footnote 6 "Derivative Financial Instruments" for information related to the offsetting of derivatives in the Company's Consolidated Financial Statements.
Maturities of Securities loaned and Financial instruments sold under agreements to repurchase are provided in the table below (dollars in thousands):


As of March 31, 2015	Overnight				0 - 30 days				31 - 60 days				61 - 90 days				Total
Securities loaned	$	792,887			$	—			$	—			$	—			$	792,887

Financial instruments sold under agreements to repurchase	85,567				445,000				225,000				150,000				905,567

Total	$	878,454			$	445,000			$	225,000			$	150,000			$	1,698,454



As of December 31, 2014	Overnight				0 - 30 days				31 - 60 days				61 - 90 days				Total
Securities loaned	$	707,744			$	—			$	—			$	—			$	707,744

Financial instruments sold under agreements to repurchase	73,576				410,000				325,000				125,000				933,576

Total	$	781,320			$	410,000			$	325,000			$	125,000			$	1,641,320

Receivable_from_and_Payable_to

Receivable from and Payable to Brokers, Dealers and Clearing Organizations	3 Months Ended
	Mar. 31, 2015
Brokers and Dealers [Abstract]
Receivable from and Payable to Brokers, Dealers and Clearing Organizations	Receivable from and Payable to Brokers, Dealers and Clearing Organizations
	Amounts receivable from and payable to brokers, dealers and clearing organizations consist of the following (in thousands):


		March 31,		December 31, 2014
		2015
	Receivable:
	Clearing organizations and other	$	622,225	$	1,073,480

	Financial instruments purchased under agreement to resell	133,935		21,545

	Securities failed to deliver	162,346		93,808

	Total Receivable	$	918,506	$	1,188,833

	Payable:
	Clearing organizations and other	$	485,866	$	350,627

	Securities failed to receive	52,977		325,462

	Total Payable	$	538,843	$	676,089

	Management believes that the carrying value of amounts receivable from and payable to brokers, dealers and clearing organizations approximates fair value since they are short term in nature.

Investments

Investments	3 Months Ended
	Mar. 31, 2015
Investments [Abstract]
Investments	Investments
	Investments primarily comprise strategic investments and deferred compensation investments. Investments consist of the following (in thousands):


		March 31,		December 31,
		2015		2014
	Strategic investments:
	Investments accounted for under the equity method	$	90,369	$	86,328

	Investments held at fair value	4,738		4,435

	Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	8,948		8,949

	Total Strategic investments	104,055		99,712

	Other investments	1,569		1,014

	Total Investments	$	105,624	$	100,726

	For the three months ended March 31, 2015 and 2014, the Company recorded income of $2.2 million and $13.0 million, respectively, related to Investments accounted for under the equity method of accounting, which is recorded within Investment income and other, net in the Consolidated Statements of Operations. The majority of the Company's income from investments accounted for under the equity method was from investees which are considered to be related parties. See Footnote 12 "Related Parties".
	Investments held at fair value are accounted for as available for sale securities and any unrealized gains or losses are recorded net of tax in Other comprehensive income.
	Merger of BATS and Direct Edge
	In January 2014, BATS and Direct Edge, each of whose equity the Company held as an investment, merged, with BATS being the surviving entity in the merger. Prior to the merger, the Company accounted for its investment in BATS under the cost method and accounted for its investment in Direct Edge under the equity method. Following the merger, the Company owns 16.7% of the overall equity of BATS and holds 19.9% of the voting equity and has appointed a director to BATS' board of directors. Based on these facts, the Company accounts for its interest in BATS under the equity method.
	The Company received approximately $42.2 million from the aggregate distributions paid by BATS and Direct Edge at or around the close of the merger, which the Company recorded as a return of capital under the equity method of accounting.
	During the first quarter of 2014 the Company recognized income of $9.6 million related to the merger of BATS and Direct Edge which is recorded within Investment income and other, net in the Consolidated Statements of Operations. The $9.6 million comprises a partial realized gain with respect to the Company's investment in Direct Edge of $16.2 million offset, in part, by the Company's share of BATS' and Direct Edge's merger related transaction costs that were charged against their earnings of $6.6 million.

Goodwill_and_Intangible_Assets

Goodwill and Intangible Assets	3 Months Ended
	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets
	Goodwill and intangible assets with indefinite lives are assessed for impairment annually or when events indicate that the amounts may be impaired. The Company assesses goodwill for impairment at the reporting unit level. The Company’s reporting units are the components of its business segments for which discrete financial information is available and is regularly reviewed by the Company’s management. As part of the assessment for impairment, the Company considers the cash flows of the respective reporting unit and assesses the fair value of the respective reporting unit as well as the overall market value of the Company compared to its net book value. The assessment of fair value of the reporting units is principally performed using a discounted cash flow methodology with a risk-adjusted weighted average cost of capital which the Company believes to be the most reliable indicator of the fair values of its respective reporting units. The Company also assesses the fair value of each reporting unit based upon its estimated market value and assesses the Company’s overall market value based upon the market price of KCG Class A Common Stock.
	Intangible assets are assessed for recoverability when events or changes in circumstances indicate that the carrying amount of the asset or asset group may not be recoverable. The Company assesses intangible assets for impairment at the “asset group” level which is the lowest level for which identifiable cash flows are largely independent of the cash flows of other groups of assets and liabilities. As part of the assessment for impairment, the Company considers the cash flows of the respective asset group and assesses the fair value of the respective asset group. Step 1 of the impairment assessment for intangibles is performed using undiscounted cash flow models, which indicates whether the future cash flows of the asset group are sufficient to recover the book value of such asset group. When an asset is not considered to be recoverable, step 2 of the impairment assessment is performed using a discounted cash flow methodology with a risk-adjusted weighted average cost of capital to determine the fair value of the intangible asset group. In cases where amortizable intangible assets and goodwill are assessed for impairment at the same time, the amortizable intangibles are assessed for impairment prior to goodwill being assessed.
	No events occurred in the three months ended March 31, 2015 or 2014 that would indicate that the carrying amounts of the Company’s goodwill or intangible assets may not be recoverable. In the fourth quarter of 2014, the Company assessed the impairment of goodwill and intangible assets as part of its annual assessment and concluded that there was no impairment.
	The following table summarizes the Company’s goodwill by segment (in thousands):


		March 31,				December 31, 2014
		2015
	Market Making	$	16,404			$	16,404

	Global Execution Services	907				907

	Total	$	17,311			$	17,311

	Intangible assets with definite useful lives are amortized over their estimated remaining useful lives, the majority of which have been determined to range from one to 8 years. The weighted average remaining life of the Company’s intangible assets with definite useful lives at March 31, 2015 and December 31, 2014 was approximately five years.
	The following tables summarize the Company’s Intangible assets, net of accumulated amortization by segment and type (in thousands):


		March 31,				December 31, 2014
		2015
	Market Making
	Technology	$	48,090			$	50,542

	Trading rights	43,191				44,358

	Total	91,281				94,900

	Global Execution Services (1)
	Technology	16,900				18,200

	Customer relationships	10,472				10,833

	Trade names	825				850

	Total	28,197				29,883

	Corporate and Other
	Technology	9,750				10,500

	Total	$	129,228			$	135,283


	(1)	Excluded from the December 31, 2014 balance is $34.7 million of intangibles related to the KCG Hotspot which was held for sale. As noted in Footnote 3 "Sales of businesses", such amount is included in Assets of business held for sale at December 31, 2014.


			March 31,				December 31, 2014
			2015
	Technology (1)	Gross carrying amount	$	117,729			$	115,804

		Accumulated amortization	(42,989		)		(36,562		)
		Net carrying amount	74,740				79,242

	Trading rights (2)	Gross carrying amount	62,468				62,468

		Accumulated amortization	(19,277		)		(18,110		)
		Net carrying amount	43,191				44,358

	Customer relationships (3)	Gross carrying amount	13,000				13,000

		Accumulated amortization	(2,528		)		(2,167		)
		Net carrying amount	10,472				10,833

	Trade names (4)	Gross carrying amount	1,000				1,000

	Accumulated amortization	(175		)		(150		)
	Net carrying amount	825				850

Total	Gross carrying amount	194,197				192,272

	Accumulated amortization	(64,969		)		(56,989		)
	Net carrying amount	$	129,228			$	135,283


(1)	The weighted average remaining life for technology, including capitalized software, was approximately 3 years for both March 31, 2015 and December 31, 2014. Excluded from the December 31, 2014 balance is $13.1 million of technology assets related to KCG Hotspot which as noted in Footnote 3 "Sales of businesses", is included in Assets of business held for sale at December 31, 2014.

(2)	Trading rights provide the Company with the rights to trade on certain exchanges. The weighted average remaining life of trading rights with definite useful lives was approximately 7 years for both March 31, 2015 and December 31, 2014. As of March 31, 2015 and December 31, 2014, $6.8 million of trading rights had indefinite useful lives.

(3)	Customer relationships relate to KCG BondPoint. The weighted average remaining life was approximately 7 and 8 years as of March 31, 2015 and December 31, 2014, respectively. Lives may be reduced depending upon actual retention rates. Excluded from the December 31, 2014 balance is $19.0 million of customer relationships related to KCG Hotspot which as noted in Footnote 3 "Sales of businesses", is included in Assets of business held for sale at December 31, 2014.

(4)	Trade names relate to KCG BondPoint. The weighted average remaining life was approximately 8 and 9 years as of March 31, 2015 and December 31, 2014, respectively. Excluded from the December 31, 2014 balance is $2.6 million of the trade name related to KCG Hotspot which as noted in Footnote 3 "Sales of businesses", is included in Assets of business held for sale at December 31, 2014.
The following table summarizes the Company’s amortization expense from continuing operations relating to Intangible assets (in thousands):


	For the three months ended March 31,
	2015				2014
Amortization expense	$	9,249			$	8,494

As of March 31, 2015, the following table summarizes the Company’s estimated amortization expense for future periods (in thousands):


	Amortization
	expense
For the nine months ended December 31, 2015	$	23,671

For the year ended December 31, 2016	30,679

For the year ended December 31, 2017	27,892

For the year ended December 31, 2018	15,788

For the year ended December 31, 2019	6,211

Debt

Debt	3 Months Ended
	Mar. 31, 2015
Debt Disclosure [Abstract]
Debt	Debt
	The carrying value and fair value of the Company's debt is as follows (in thousands):


		March 31, 2015						December 31, 2014
		Carrying Amount				Fair Value		Carrying Amount		Fair Value
	Cash Convertible Senior Subordinated Notes	$	—			$	—	$	117,259	$	116,819

	8.25% Senior Secured Notes	305,000				329,747		305,000		309,194

	6.875% Senior Secured Notes	494,847				484,950		—		—

	Total Debt	$	799,847			$	814,697	$	422,259	$	426,013

	The fair value of the Company's 6.875% Senior Secured Notes and Cash Convertible Senior Subordinated Notes are based upon the value of such debt in the secondary market.
	The fair value of the $305.0 million 8.25% Senior Secured Notes represents the settlement amount of $329.7 million paid to the holders of these Notes as discussed in further detail below.
	All of the above liabilities would be categorized as Level 2 in the fair value hierarchy if they were required to be recorded at fair value.
	Cash Convertible Senior Subordinated Notes
	In March 2010, Knight issued $375.0 million aggregate principal amount of Cash Convertible Senior Subordinated Notes (the “Convertible Notes”), due on March 15, 2015, in a private offering exempt from registration under the Securities Act of 1933, as amended (the "Securities Act"). In connection with the closing of the Mergers, on July 1, 2013, KCG became a party to the indenture under which the which the Convertible Notes were issued.
	On March 16, 2015, the Convertible Notes became due and were paid off in full with a payment of $119.3 million comprising $117.3 million in principal and $2.1 million in interest.
	The Convertible Notes bore interest at a rate of 3.50% per year, payable semi-annually in arrears, on March 15 and September 15 of each year, commencing on September 15, 2010.
	8.25% Senior Secured Notes
	On June 5, 2013 GETCO Financing Escrow LLC (“Finance LLC”), a wholly-owned subsidiary of GETCO, issued 8.250% senior secured notes due 2018 in the aggregate principal amount of $305.0 million (the “8.25% Senior Secured Notes”) pursuant to an indenture, dated June 5, 2013 (as amended and supplemented, the "8.25% Senior Secured Notes Indenture"). On July 1, 2013, KCG entered into a first supplemental indenture (the “First Supplemental Indenture”) pursuant to which KCG assumed all of the obligations of Finance LLC which comprised the 8.25% Senior Secured Notes plus certain escrow agent fees and expenses of $3.0 million.
	The 8.25% Senior Secured Notes were scheduled to mature on June 15, 2018 and bore interest at a rate of 8.250% per year, payable on June 15 and December 15 of each year, beginning on December 15, 2013.
	The 8.25% Senior Secured Notes Indenture provided that KCG could redeem the 8.25% Senior Secured Notes, in whole or in part, at any time prior to June 15, 2015 at a price equal to 100% of the aggregate principal amount of the 8.25% Senior Secured Notes to be redeemed, plus a make-whole premium and accrued and unpaid interest, if any.
	On March 13, 2015, KCG provided 30 days’ notice that it would be calling its existing 8.25% Senior Secured Notes, effective April 13, 2015. On March 13, 2015, the Company used a portion of the gross proceeds from the 6.875% Senior Secured Notes (as defined below), to deposit in an escrow account maintained by The Bank of New York Mellon, the trustee of the 8.25% Senior Secured Notes (“Bank of New York”) an amount sufficient to redeem the 8.25% Senior Secured Notes in full and accordingly satisfied and discharged the 8.25% Senior Secured Notes Indenture. The Company funded $330.2 million into an escrow account maintained by Bank of New York comprising the following: principal of $305.0 million, accrued interest for the period from December 16, 2014 to April 13, 2015 of $8.2 million, make whole premium which includes 4.125% early redemption cost plus additional interest due from April 13, 2015 through June 15, 2015 totaling $16.5 million, and additional funds to cover other miscellaneous charges of $0.4 million. As discussed in Footnote 22, “Subsequent Events”, on April 13, 2015, the escrow amount of $330.2 million was released and $329.7 million was paid out to the 8.25% Senior Secured Notes holders to redeem the 8.25% Senior Secured Notes.
	The 8.25% Senior Secured Notes Indenture contained customary affirmative and negative covenants, including limitations on indebtedness, liens, hedging agreements, investments, loans and advances, asset sales, mergers and acquisitions, dividends, transactions with affiliates, prepayments of other indebtedness, restrictions on subsidiaries and issuance of capital stock. As of December 31, 2014 the Company was in compliance with the covenants.
	6.875% Senior Secured Notes
	On March 10, 2015, the Company entered into a purchase agreement with Jefferies LLC, as initial purchaser (the “Initial Purchaser”), pursuant to which the Company agreed to sell, and the Initial Purchaser agreed to purchase, $500.0 million aggregate principal amount of 6.875% Senior Secured Notes (the “6.875% Senior Secured Notes ”), pursuant to an indenture dated, March 13, 2015 (the “6.875% Indenture”), in a private offering exempt from registration under the Securities Act. The 6.875% Senior Secured Notes were resold by the Initial Purchaser to qualified institutional buyers in reliance on Rule 144A and Regulation S under the Securities Act.
	The 6.875% Senior Secured Notes mature on March 15, 2020 and bear interest at a rate of 6.875% per year, payable on March 15 and September 15 of each year, beginning on September 15, 2015. The 6.875% Senior Secured Notes were issued at 98.962% with net proceeds (before fees and expenses) of $494.8 million and a yield to maturity of 7.083%.
	On March 13, 2015, KCG and certain subsidiary guarantors (the "6.875% Guarantors") under the 6.875% Indenture, fully and unconditionally guaranteed on a joint and several basis the 6.875% Senior Secured Notes. The 6.875% Senior Secured Notes and the obligations under the 6.875% Indenture are currently secured by pledges of all of the equity interests in each of KCG’s and the 6.875% Guarantors’ existing and future domestic subsidiaries (but limited to 66% of the voting equity interests of controlled foreign company subsidiaries and excluding equity interests in regulated subsidiaries to the extent that such pledge would have a material adverse regulatory effect or is not permitted by applicable law) and security interests in substantially all other tangible and intangible assets of KCG and the 6.875% Guarantors, in each case subject to customary exclusions; provided, however, that if in the future KCG or any of the 6.875% Guarantors enter into certain first lien obligations (as described in the 6.875% Indenture) the collateral agent is authorized by the holders of the 6.875% Senior Secured Notes to enter into an Intercreditor Agreement pursuant to which the lien securing the 6.875% Senior Secured Notes would be contractually subordinated to the lien securing such first lien obligations, to the extent of the value of the collateral securing such obligations. The 6.875% Senior Secured Notes are effectively subordinated to any existing and future indebtedness that is secured by assets that do not constitute collateral under the 6.875% Senior Secured Notes to the extent of the value of such assets. All of the 6.875% Guarantors are wholly-owned subsidiaries of KCG.
	On or after March 15, 2017, KCG may redeem all or a part of the 6.875% Senior Secured Notes upon not less than 30 nor more than 60 days’ notice, at the redemption prices (expressed as percentages of principal amount) set forth below plus accrued and unpaid interest and additional interest on the 6.875% Senior Secured Notes redeemed, to the applicable redemption date, if redeemed during the 12-month period beginning on March 15 of the years indicated below:


	Year		Percentage
	2017		103.438	%
	2018		101.719	%
	2019 and thereafter		100	%
	KCG may also redeem the 6.875% Senior Secured Notes, in whole or in part, at any time prior to March 15, 2017 at a price equal to 100% of the aggregate principal amount of the 6.875% Senior Secured Notes to be redeemed, plus a make-whole premium and accrued and unpaid interest. In addition, at any time on or prior to March 15, 2017, KCG may redeem up to 40% of the aggregate principal amount of the 6.875% Senior Secured Notes with the net cash proceeds of certain equity offerings, at a price equal to 106.875% of the aggregate principal amount of the 6.875% Senior Secured Notes, plus accrued and unpaid interest, if any.
	The 6.875% Indenture contains customary affirmative and negative covenants, including limitations on indebtedness, liens, hedging agreements, investments, loans and advances, asset sales, mergers and acquisitions, dividends, transactions with affiliates, prepayments of other indebtedness, restrictions on subsidiaries, and issuance of capital stock. As of March 31, 2015, the Company was in compliance with the covenants.
	If at any time the 6.875% Senior Secured Notes are rated investment grade by Moody’s Investors Service, Inc. and Standard & Poor’s Ratings Group and no default or event of default has occurred and is continuing under the 6.875% Indenture, certain of the restrictive covenants will be suspended and will not apply to KCG or its restricted subsidiaries; provided, however, that such covenants will be reinstated if the 6.875% Senior Secured Notes subsequently cease to be rated or are no longer assigned an investment grade rating by both rating agencies.
	The 6.875% Senior Secured Notes and the guarantee of the 6.875% Senior Secured Notes have not been registered under the Securities Act or the securities laws of any other jurisdiction and have no registration rights and may not be offered or sold in the United States absent registration or an applicable exemption from registration requirements.
	The Company has determined that the terms of the 6.875% Senior Secured Notes do not give rise to a bifurcatable derivative instrument under GAAP.
	The Company incurred issuance costs of approximately $12.4 million in connection with the issuance of 6.875% Senior Secured Notes. The issuance costs are recorded within Other assets on the Consolidated Statements of Financial Condition and are being amortized over the remaining term of the 6.875% Senior Secured Notes. Including issuance costs, the 6.875% Senior Secured Notes had an effective yield of 7.580%.
	First Lien Credit Facility
	On July 1, 2013, KCG, as borrower, entered into a first lien senior secured credit agreement (the “Credit Agreement”) with Jefferies Finance LLC and Goldman Sachs Bank USA. The Credit Agreement was in the amount of $535.0 million (the “First Lien Credit Facility”), all of which was drawn on July 1, 2013. The First Lien Credit Facility also provided for a future uncommitted incremental first lien senior secured revolving credit facility of up to $50.0 million, including letter of credit and swingline sub-facilities, on certain terms and conditions contained in the Credit Agreement.
	In 2013, the Company repaid $300.0 million of principal of the First Lien Credit Facility. A portion of the $300.0 million totaling $117.3 million was drawn from cash held in the Collateral Account and the remainder of the $300.0 million was paid out of available cash, including proceeds from the sale of Urban.
	In the first quarter of 2014, the Company repaid $185.0 million of principal of the First Lien Credit Facility out of available cash. In conjunction with these payments, the Company wrote down $7.6 million of its capitalized debt costs associated with the Credit Agreement. In the second quarter of 2014, the Company made a final $50.0 million payment of principal on the First Lien Credit Facility out of available cash and the Credit Agreement was terminated.
	Revolving Credit Agreement
	On July 1, 2013, OCTEG, LLC (“OCTEG”) and Knight Capital Americas LLC ("KCA"), wholly-owned broker dealer subsidiaries of KCG, as borrowers, and KCG, as guarantor, entered into a credit agreement (the "KCGA Facility Agreement”) with a consortium of banks and financial institutions. The KCGA Facility Agreement replaced an existing credit agreement, dated as of June 6, 2012, among OCTEG and three banks.
	The KCGA Facility Agreement comprises two classes of revolving loans in a total committed amount of $450.0 million, together with a swingline facility with a $50.0 million sub-limit, subject to two borrowing bases (collectively, the “KCGA Revolving Facility”): Borrowing Base A and Borrowing Base B. The KCGA Revolving Facility also provides for a future increase of the revolving credit facility of up to $300.0 million to a total of $750.0 million on certain terms and conditions.
	The KCGA Revolving Facility was amended on October 24, 2013 to permit OCTEG to be removed as a borrower under the KCGA Revolving Facility. As of January 1, 2014, OCTEG was merged with and into KCA and KCA was renamed KCG Americas LLC ("KCGA").
	Borrowings under the KCGA Revolving Facility bear interest, at the borrower's option, at a rate based on the federal funds rate (“Base Rate Loans”) or based on LIBOR (“Eurodollar Loans”), in each case plus an applicable margin. For each Base Rate Loan, the interest rate per annum is equal to the greater of the federal funds rate or an adjusted one-month LIBOR rate plus (a) for each Borrowing Base A loan, a margin of 1.75% per annum and (b) for each Borrowing Base B loan, a margin of 2.25% per annum. For each Eurodollar Loan, the interest rate per annum is equal to an adjusted LIBOR rate corresponding to the interest period plus (a) for each Borrowing Base A loan, a margin of 1.75% per annum and (b) for each Borrowing Base B loan, a margin of 2.25% per annum. As of March 31, 2015 and December 31, 2014, there were no outstanding borrowings under the KCGA Facility Agreement.
	The proceeds of the Borrowing Base A loans may be used solely to finance the purchase and settlement of securities. The proceeds of Borrowing Base B loans may be used solely to fund clearing deposits with the National Securities Clearing Corporation ("NSCC").
	The borrower is being charged a commitment fee at a rate of 0.35% per annum on the average daily amount of the unused portion of the KCGA Facility Agreement.
	The loans under the KCGA Facility Agreement will mature on June 6, 2015. The KCGA Revolving Facility is fully and unconditionally guaranteed on an unsecured basis by KCG and, to the extent elected by KCGA, any of their respective subsidiaries. It is secured by first-priority pledges of and liens on certain eligible securities, subject to applicable concentration limits, in the case of Borrowing Base A loans, and by first-priority pledges of and liens on the right to the return of certain eligible NSCC margin deposits, in the case of Borrowing Base B loans.
	The KCGA Facility Agreement includes customary affirmative and negative covenants, including limitations on indebtedness, liens, hedging agreements, investments, loans and advances, asset sales, mergers and acquisitions, dividends, transactions with affiliates, restrictions on subsidiaries, issuance of capital stock, negative pledges and business activities. It contains financial maintenance covenants establishing a minimum total regulatory capital for KCGA, a maximum total asset to total regulatory capital ratio for KCGA, a minimum excess net capital limit for KCGA, a minimum liquidity ratio for KCGA, and a minimum tangible net worth threshold for KCGA. As of March 31, 2015 and December 31, 2014, the Company was in compliance with the covenants.
	In connection with the KCGA Revolving Facility, the Company incurred issuance costs of $1.2 million which is recorded within Other assets on the Consolidated Statements of Financial Condition and it is being amortized over the term of the facility.
	The Company is seeking to renew the existing KCGA Facility Agreement under similar terms, which is dependent on current credit market conditions. As of March 31, 2015, the final structure and terms has not yet been finalized.
	The Company recorded expenses with respect to the Debt as follows (in thousands):


		For the three months ended March 31,
		2015				2014
Interest expense	$	8,158			$	9,416

Amortization of debt issuance cost (1)	934				8,766

Commitment fee	394				397

Total	$	9,486			$	18,579


(1)	For the three months ended March 31, 2014, $7.6 million of amortization of debt issuance cost incurred is included in Writedown of debt issuance costs while $1.6 million which includes the remaining amortization of the debt issuance costs and the commitment fee is included in Other expenses.

Related_Parties

Related Parties	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Related Parties	Related Parties
	The Company interacts with two parties which are the beneficial owners of more than 10 percent of KCG’s Class A Common Stock. It also has trading and other activities with four investees whereby KCG accounts for its investment under the equity method of accounting. Each is considered a related party for all applicable periods. See Footnote 9 "Investments" for the carrying value of these investees at March 31, 2015 and 2014 and for the Company's income with respect to its equity earnings from these investees for the three months ended March 31, 2015 and 2014.
	The Company earns revenues, incurs expenses and maintains balances with these related parties or their affiliates in the ordinary course of business. As of the date and period indicated below, the Company had the following balances and transactions with the related parties or their affiliates (in thousands):


		For the three months ended March 31,
	Statements of Operations	2015			2014
	Revenues
	Commissions and fees	$	3,492		$	5,146

	Trading revenues, net	2,101			814

	Interest, net	226			188

	Total revenues from related parties	$	5,819		$	6,148

	Expenses
	Execution and clearance fees(1)	$	(4,628	)	$	(4,011	)
	Communications and data processing	1,110			—

	Payment for order flow	1,180			—

	Collateralized financing interest	113			159

	Professional fees	5,507			—

	Other expense	621			400

	Total expenses incurred with respect to related parties	$	3,903		$	(3,452	)

	(1) Represents net volume based fees received from providing liquidity to related trading venues.


	Statements of Financial Condition	March 31,			December 31,
		2015			2014
	Assets
	Securities borrowed	$	41,600		$	26,110

	Receivable from brokers, dealers and clearing organizations	92,739			20,075

	Other assets	12,247			—

	Liabilities
	Securities loaned	$	3,543		$	7,376

	Payable to brokers, dealers and clearing organizations	64,757			8,509

	Accrued expenses and other liabilities	4,693			5,667

	In March 2015, the Company completed the sale of KCG Hotspot to BATS, a related party. The Company recorded a gain on sale of $385.0 million which is recorded as Investment income and other, net on the Consolidated Statements of Operations for the three months ended March 31, 2015.
	Additionally, in the first quarter of 2015, the Company paid one of the related parties $16.8 million in fees related to financing and advisory activities. The $16.8 million comprised $11.3 million that was capitalized as deferred debt costs within Other assets on the Consolidated Statement of Financial Condition and $5.5 million that was recorded as Professional fees in the Consolidated Statement of Operations.
	In the ordinary course of business, the Company enters into foreign exchange contracts with related parties.

StockBased_Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Stock-Based Compensation
	KCG Equity Incentive Plan
	KCG Holdings, Inc. Amended and Restated Equity Incentive Plan (the "KCG Plan") was initially assumed from Knight in connection with the Mergers, and since, has been maintained by the Company for the purpose of granting long-term incentive awards to officers, employees and directors of the Company. As of March 31, 2015, there were approximately 28.9 million shares authorized for issuance under the KCG Plan, of which approximately 15.1 million shares are available for grant (subject to adjustment as provided under the KCG Plan).
	The KCG Plan is administered by the Compensation Committee of the Company’s Board of Directors (the "Compensation Committee"), and allows for the grant of stock options, SARs, restricted stock and RSUs (collectively, the “awards”), as defined by the KCG Plan. In addition to overall limitations on the aggregate number of awards that may be granted, the KCG Plan also limits the number of awards that may be granted to a single individual.
	Restricted Shares and Restricted Stock Units
	The Company has historically awarded RSUs to eligible officers, employees and directors as a component of annual incentive compensation, and has also made off-cycle grants of RSUs for purposes of one-time, special or retention awards. The majority of RSUs that have been granted by the Company vest ratably over three years and are subject to accelerated vesting, or continued vesting, following the grantee’s termination of employment, in accordance with the applicable award documents and employment agreements between the Company and the grantee. Officers, employees and directors generally become vested in outstanding RSUs upon a qualifying retirement, and the KCG Plan otherwise provides the Compensation Committee with the discretion to fully vest participants in outstanding RSUs in certain other circumstances.
	The Company measures compensation cost related to RSUs on the fair value of KCG Class A Common Stock at the date of grant. Compensation expense from continuing operations relating to RSUs, which is primarily recorded in Employee compensation and benefits, and the corresponding income tax benefit, which is recorded in Income tax expense on the Consolidated Statements of Operations are presented in the following table (in thousands):


		For the three months ended March 31,
		2015				2014
	Stock award compensation expense	$	14,255			$	16,238

	Income tax benefit	5,417				6,170

	The following table summarizes restricted awards activity for the three months ended March 31, 2015 (awards in thousands):


			Restricted Stock Units
			Number of			Weighted-
			Units			Average
						Grant date
						Fair Value
	Outstanding at December 31, 2014		9,147			$	10.77

	Granted		2,385			12.22

	Vested		(2,055	)		11.04

	Forfeited		(322	)		11.25

	Outstanding at March 31, 2015		9,155			$	11.07

	There is $59.2 million of unamortized compensation related to unvested RSUs outstanding at March 31, 2015. The cost of these unvested RSUs, unless a modification occurs, is expected to be recognized over a weighted average life of 1.64 years.
	Stock Options and Stock Appreciation Rights
	The Company’s policy is to grant options for the purchase of shares of KCG Class A Common Stock and SARs to purchase or receive the cash value of shares of KCG Class A Common Stock. The stock options and SARs outstanding as of the date hereof have each been granted with an exercise price not less than the market value of KCG Class A Common Stock on the grant date and generally vest ratably over a three year period and expire on the fifth or tenth anniversary of the grant date, pursuant to the terms of the applicable award agreement. Like RSUs, stock options and SARs are subject to accelerated vesting, or continued vesting following certain termination circumstances, in accordance with the applicable award agreements and employment agreements between the Company and the participant. The Company issues new shares upon stock option exercises by its employees and directors, and may either issue new shares or provide a cash payment upon SARs exercises by its employees.
	The Company estimates the fair value of each stock option and SAR granted as of its respective grant date using the Black-Scholes option-pricing model. The principal assumptions utilized in valuing stock options and SARs and the methodology for estimating the inputs to such model include: 1) risk-free interest rate, the estimate of which is based on the yield of U.S. zero coupon securities with a maturity equal to the expected life of the stock option or SAR; 2) expected volatility, the estimate of which is based on several factors including implied volatility of market-traded stock options on the Company’s common stock on the grant date and the volatility of the Company’s common stock; and 3) expected option or SAR life, the estimate of which is based on internal studies of historical experience and projected exercise behavior based on different employee groups and specific stock option and SAR characteristics, including the effect of employee terminations. There were no stock options or SARs granted during the three months ended March 31, 2015 or 2014.
	Compensation expense from continuing operations relating to stock options and SARs, all of which was recorded in Employee compensation and benefits, as well as the corresponding income tax benefit, which is recorded in Income tax expense on the Consolidated Statements of Operations are as follows (in thousands):


		For the three months ended March 31,
		2015				2014
	Stock option and SAR compensation expense	$	1,013			$	961

	Income tax benefit	385				365

	The following table summarizes stock option and SAR activity and stock options exercisable for the three months ended March 31, 2015 (awards in thousands):


			Number of Stock Awards			Weighted-		Aggregate		Weighted-
						Average		Intrinsic		Average
						Exercise		Value		Remaining
						Price				Life (years)
	Outstanding at December 31, 2014 (1)		4,691			$	17.41

	Granted at market value		—			—

	Exercised		—			—

	Forfeited or expired		(84	)		17.1

	Outstanding at March 31, 2015 (1)		4,607			$	17.42	$	4,687	3.15

	Exercisable at March 31, 2015		1,782			$	20.72	$	1,640	2.95

Available for future grants at March 31, 2015 (2)		15,086

(1) Includes 1.7 million of SARs.
(2) Represents options, SARs and awards available for grant under the KCG Plan.
The aggregate intrinsic value is the amount by which the closing price of the Company’s Class A Common Stock exceeds the exercise price of the stock options multiplied by the number of shares. There were no stock options or SARs exercised during the three months ended March 31, 2015 and 2014.
There is $2.3 million of unamortized compensation related to unvested stock options and SARs outstanding at March 31, 2015. The cost of these unvested awards is expected to be recognized over a weighted average life of 0.65 years.
Incentive units
Prior to the Mergers, GETCO awarded deferred compensation to its employees in the form of incentive units that generally vested over time. The value of these incentive units was determined at the date of grant based on the estimated enterprise value of GETCO and the amount expensed was determined based on this valuation multiplied by the percent vested. In connection with the Mergers, all outstanding unvested incentive units vested and were converted into units based on the applicable exchange ratio of GETCO units to the Company's Class A Common Stock. The units are marked to the current stock price of the Company's Class A Common Stock at the end of each period with the resulting change in the liability reflected as either an expense or gain included in Employee compensation and benefits on the Consolidated Statements of Operations. Given that the units vested in connection with the Mergers, the Company fully amortized the costs associated with these units as of June 30, 2013. Deferred compensation payable at March 31, 2015 and December 31, 2014 related to incentive units was $3.1 million and $2.9 million, respectively, and is included in Accrued compensation expense on the Consolidated Statements of Financial Condition.
The following is a summary of the changes in the incentive units for the three months ended March 31, 2015 (units in thousands):


	Vested
Incentive units at December 31, 2014	38

Issued	—

Vested	—

Exercised	—

Canceled	—

Incentive units at March 31, 2015	38

Compensation expense (benefit) related to the Incentive units which are recorded within Employee compensation and benefits on the Consolidated Statements of Operations are as follows (in thousands):


	For the three months ended March 31,
	2015				2014
Incentive units	$	153			$	(174	)

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	The Company and its subsidiaries will file a consolidated federal income tax return as well as combined state income tax returns in certain jurisdictions. In other jurisdictions, the Company and its subsidiaries will file separate company state and local income tax returns.
	The following table reconciles the U.S. federal statutory income tax rate to the Company's actual income tax rate from continuing operations (in thousands):


		For the three months ended March 31,
		2015		2014

	U.S. federal statutory income tax rate	35	%	35	%
	U.S. state and local income taxes, net of U.S.
	Federal income tax effect	3.6	%	2.2	%
	Nondeductible expenses(1)	0	%	0.3	%
	Foreign Taxes	0	%	0.1	%
	Other, net	0	%	0.2	%
	Actual income tax rate	38.6	%	37.8	%
	(1) Nondeductible expenses include nondeductible compensation and meals and entertainment
	Deferred income taxes reflect the net tax effects of temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when such differences are expected to reverse. Valuation allowances recorded on the balance sheet dates are necessary in cases where management believes that it is more likely than not that some portion or all of the deferred tax assets will not be realized.
	Based on the weight of the positive and negative evidence considered, management believes that it is more likely than not that the Company will be able to realize the majority of its federal deferred tax assets in the future, and except as noted below with respect to certain NOLs, no valuation allowance has been recorded at March 31, 2015 with respect to such federal deferred tax assets. Management believes that positive evidence including the Company's history of sustainable profitability, actual profitability, and its forecasts of future profitability outweighs the negative evidence. The Company has recorded a partial valuation allowance against state and local deferred tax assets as it is more likely than not that the benefit of such items will not be realized due to limitations on utilization in the particular jurisdictions in which the Company operates.
	In accordance with Section 382 of the Internal Revenue Code, a change in equity ownership of greater than 50% of a corporation within a three-year period results in an annual limitation on the corporation’s ability to utilize its net operating loss ("NOL") carryforwards that were created during tax periods prior to the change in ownership. As a result of the Mergers as well as prior ownership changes, Knight experienced ownership changes under Section 382 and as a result, the rate of utilization of NOL carryforwards generated by Knight may be limited. The Company does not believe these limitations will have a significant effect on the Company's ability to utilize its anticipated federal NOL carryforward. The Company's U.S. federal NOL carryforwards will begin to expire in 2019.
	At March 31, 2015, the Company had total U.S. federal NOL carryforwards of $133.5 million, of which $28.5 million resulted from the acquisition of Knight. At March 31, 2015 the Company recorded a deferred tax asset related to these federal NOLs of $46.7 million and a partially offsetting valuation allowance of $6.8 million which represents the portion of these NOL carryforwards that are considered more likely than not to expire unutilized.
	At March 31, 2015, the Company recorded a valuation allowance for certain of its state and local NOL carryforwards as it is more likely than not that the benefit of these carryforwards will not be realized due to limitations on utilization in the particular jurisdictions in which the Company operates. Certain of these carryforwards are subject to annual limitations on utilization and they will begin to expire in 2019.
	At March 31, 2015, the Company had, in multiple jurisdictions, aggregate state and local NOL carryforwards of $1.28 billion, all of which resulted from the acquisition of Knight. The Company recorded deferred income tax assets related to these NOLs of $18.5 million as of March 31, 2015 and a partially offsetting valuation allowances of $8.1 million, which represents the portion of these NOLs that are considered more likely than not to expire unutilized. In 2014 the Company reversed $10.8 million of the valuation related to its state and local NOLs as a portion of such loss carryforwards are now expected to be utilized based upon the expected profitability of certain of the Company’s subsidiaries. Certain of these carryforwards are subject to annual limitations on utilization and these NOLs will begin to expire in 2019.
	At March 31, 2015, the Company had non-U.S. NOL carryforwards of $86.4 million of which $65.7 million were generated by Knight in periods prior to the Mergers. The Company recorded a foreign deferred income tax asset of $19.5 million for these NOL carryforwards as of March 31, 2015 along with an offsetting U.S. federal deferred tax liability of $19.5 million for the expected future reduction in U.S. foreign tax credits associated with the use of the non-U.S. loss carryforwards. These non-U.S. net operating losses may be carried forward indefinitely.
	At March 31, 2015, the Company had U.S. Federal tax credit carryforwards comprising general business credit carryforwards of $4.4 million and alternative minimum tax credit carryforwards $6.8 million.
	As of March 31, 2015, the Company is subject to U.S. Federal income tax examinations for the tax years 2011 through 2013, and to non-U.S. income tax examinations for the tax years 2007 through 2014. In addition, the Company is subject to state and local income tax examinations in various jurisdictions for the tax years 2007 through 2013. The final outcome of these examinations is not yet determinable. However, the Company anticipates that adjustments to the unrecognized tax benefits, if any, will not result in a material change to the results of operations or financial condition.
	At March 31, 2015, the Company had $2.3 million of unrecognized tax benefits, all of which would affect the Company's effective tax rate if recognized. At March 31, 2014, the Company had $1.5 million of unrecognized tax benefits, all of which would affect the Company’s effective tax rate if recognized.
	The Company's policy for recording interest and penalties associated with audits is to record such items as a component of income or loss from continuing operations before income taxes. Penalties, if any, are recorded in Other expenses and interest paid or received is recorded in Debt interest expense and Interest, net, on the Consolidated Statements of Operations.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Accumulated Other Comprehensive Income	Accumulated Other Comprehensive Income
	The following table presents changes in Accumulated other comprehensive income, net of tax by component for the three months ended March 31, 2015 and 2014 (in thousands):


		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments			Total
	Balance December 31, 2014	$	352		$	1,781		$	2,133

	Other comprehensive income (loss)	188			(623		)	(435		)

	Balance March 31, 2015	$	540		$	1,158		$	1,698



		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments			Total
	Balance December 31, 2013	$	36		$	1,365		$	1,401

	Other comprehensive income (loss)	(137		)	201			64

	Balance March 31, 2014	$	(101	)	$	1,566		$	1,465

Writedowns_and_Other_Charges

Writedowns and Other Charges	3 Months Ended
Writedowns and Other Charges	Mar. 31, 2015
Restructuring and Related Activities [Abstract]
Writedowns and Other Charges	Writedowns and Other Charges
	Writedown of capitalized debt costs
	During the first quarter of 2014, the Company made principal repayments under the Credit Agreement of $185.0 million. As a result, $7.6 million in capitalized debt costs were written off.

Earnings_Per_Share

Earnings Per Share	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share
	Basic earnings or loss per common share (“EPS”) has been calculated by dividing net income from continuing operations by the weighted average shares of the Company's Class A Common Stock outstanding during each respective period. Diluted EPS reflects the potential reduction in EPS using the treasury stock method to reflect the impact of common stock equivalents if stock options, SARs and warrants to purchase shares of the Company's Class A Common Stock were exercised and restricted awards were to vest.
	The number of such RSUs, options, warrants and SARs excluded from the EPS calculation was approximately 18.3 million and 28.4 million for the three months ended March 31, 2015 and 2014, respectively. Such options, warrants and SARS were excluded from the EPS calculation as their inclusion would have an anti-dilutive impact on the EPS calculation. The computation of diluted shares can vary among periods due in part to the change in the average price of the Company's Class A Common Stock.
	The following is a reconciliation of the numerators and denominators of the basic and diluted earnings per share computations from continuing operations for the three months ended March 31, 2015 and 2014 (in thousands):


		For the three months ended March 31,
		2015				2014
		Numerator /		Denominator /		Numerator /		Denominator /
		net income		shares		net income		shares
	Income from continuing operations and shares used in basic calculations	$	249,301	110,782		$	36,917	115,569

	Effect of dilutive stock based awards
	Restricted awards			2,410				2,233

	Stock options and SARs			286				96

	Warrants			137				—

	Income from continuing operations and shares used in diluted calculations	$	249,301	113,615		$	36,917	117,898

	Basic earnings per common share from continuing operations			$	2.25			$	0.32

	Diluted earnings per common share from continuing operations			$	2.19			$	0.31

Significant_Clients

Significant Clients	3 Months Ended
Significant Clients	Mar. 31, 2015
Risks and Uncertainties [Abstract]
Significant Clients	Significant Clients
Significant Clients	The Company considers significant clients to be those clients who account for 10% or more of the total U.S. equity market making dollar value traded by the Company. No clients accounted for more than 10% of the Company’s U.S. equity dollar value traded during the three months ended March 31, 2015 or 2014.

Commitments_and_Contingent_Lia

Commitments and Contingent Liabilities	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingent Liabilities	Commitments and Contingent Liabilities
	Legal Proceedings
	In the ordinary course of business, the nature of the Company’s business subjects it to claims, lawsuits, regulatory examinations and other proceedings. The Company and its subsidiaries are subject to several of these matters at the present time. Given the inherent difficulty of predicting the outcome of the litigation and regulatory matters, particularly in cases or proceedings in which substantial or indeterminate damages or fines are sought, or where cases or proceedings are in the early stages, the Company cannot estimate losses or ranges of losses for cases or proceedings where there is only a reasonable possibility that a loss may be incurred. In addition, there are numerous factors that result in a greater degree of complexity in class-action lawsuits as compared to other types of litigation. Due to the many intricacies involved in class-action lawsuits particularly in the early stages of such matters, obtaining clarity on a reasonable estimate is difficult which may call into question its reliability. There can be no assurance that these matters will not have a material adverse effect on the Company’s results of operations in any future period, and a material judgment could have a material adverse impact on the Company’s financial condition and results of operations. However, it is the opinion of management, after consultation with legal counsel that, based on information currently available, the ultimate outcome of these ordinary matters will not have a material adverse impact on the business, financial condition or operating results of the Company although they might be material to the operating results for any particular reporting period, depending, in part, upon operating results for that period. The Company carries directors and officers liability insurance coverage for potential claims, including securities actions, against the Company, Knight and GETCO and their respective directors and officers.
	As previously disclosed in KCG's and Knight's public filings, Knight experienced a technology issue at the open of trading at the NYSE on August 1, 2012. This issue was related to the installation of trading software and resulted in KCA sending numerous erroneous orders in NYSE-listed and NYSE Arca securities into the market. Thereafter, Knight was named as a defendant in two putative class action complaints (one of which was voluntarily dismissed and the other of which is described in more detail below) and one purported derivative lawsuit, all of which related to the technology issue. The purported derivative action was resolved as part of a settlement entered by the Delaware Court of Chancery in September 2014. Knight also received several derivative demand letters and/or requests for the inspection or production of certain books and records pursuant to Delaware law related to the technology issue and the raising of $400.0 million in equity financing through a convertible preferred stock offering to certain investors. The claims described in the derivative demand letters were resolved as part of the aforementioned Delaware settlement and the requests for inspection of documents have not been pursued.
	While the Company is currently unable to predict the outcome of any existing or future litigation related to the August 1, 2012 technology issue, an unfavorable outcome in one of these matters could have a material adverse effect on its financial condition or ongoing results of operations. In addition, the Company expects to incur additional expenses in defending against such litigation.
	Legal
	Litigation Related to the August 1, 2012 Technology Issue
	On October 26, 2012, Knight, its then-Chairman and Chief Executive Officer, Thomas M. Joyce, and its then-Executive Vice President, Chief Operating Officer and Chief Financial Officer, Steven Bisgay, were named as defendants in an action entitled Fernandez v. Knight Capital Group, Inc. in the U.S. District Court for the District of New Jersey, Case No. 2:12-cv-06760 (the “Fernandez Action”). Generally, this putative class action complaint alleged that the defendants made material misstatements and/or failed to disclose matters related to the events of August 1, 2012. The plaintiff asserted claims under Sections 10(b) and 20 and Rule 10b-5 of the federal securities laws, claiming that he and a purported class of Knight's stockholders who purchased Knight's Class A Common Stock between January 19, 2012 and August 1, 2012 paid an inflated price. Following the appointment of a lead plaintiff and counsel, the plaintiff filed an amended complaint on March 14, 2013, alleging generally that the defendants made material misstatements and/or failed to disclose matters related to the events of August 1, 2012. The plaintiff asserted claims under Sections 10(b) and 20 and Rule 10b-5 of the federal securities laws, claiming that it and a purported class of Knight's stockholders who purchased Knight's securities between November 30, 2011 and August 1, 2012 paid an inflated price. On May 13, 2013, Knight filed a motion to dismiss the amended complaint, which was fully briefed as of August 2013. Before the court rendered a decision on the motion to dismiss, the plaintiff filed a second amended complaint on December 20, 2013, alleging generally that the defendants made material misstatements and/or failed to disclose matters related to the events of August 1, 2012. More specifically, the plaintiff referred to KCA's October 2013 settlement with the SEC and alleged that the defendants made false and misleading statements concerning Knight's risk management procedures and protocols, available cash and liquidity, Value at Risk and internal controls over financial reporting. The plaintiff asserted claims under Sections 10(b) and 20 and Rule 10b-5 of the federal securities laws, claiming that it and a purported class of Knight's stockholders who purchased Knight's securities between May 10, 2011 and August 1, 2012 (the "Class Period") paid an inflated price. The defendants filed a motion to dismiss the second amended complaint on February 18, 2014 which was fully briefed as of June 5, 2014.
	In November 2014, prior to the court’s decision on defendants’ motion to dismiss, the parties participated in a court-ordered mediation. Following the mediation, in December 2014 the parties reached an agreement in principle to settle the Fernandez Action. On February 9, 2015, the parties entered into a Stipulation of Settlement that, if approved by the District Court, will resolve the litigation and result in the Fernandez Action being dismissed with prejudice. Pursuant to an Order filed on March 2, 2015, the District Court preliminarily approved the settlement of the Fernandez Action and set July 1, 2015 for the final settlement hearing. Under the terms of the proposed settlement, Knight has agreed that certain of its insurance carriers would pay $13.0 million to stockholders in the class. The settlement requires no direct payment by any of the defendants. Under the proposed settlement, defendants and various of their related persons and entities will receive a full release of all claims that were or could have been brought in the action as well as all claims that arise out of, are based upon or relate to the allegations, transactions, facts, representations, omissions or other matters involved in the complaints filed in the action or any statement communicated to the public during the Class Period, and the purchase, acquisition or sale of the Company’s stock during the Class Period. The proposed settlement contains no admission of any liability or wrongdoing on the part of the defendants, each of whom continues to deny all of the allegations against them and believes that the claims are without merit. The settlement will not have an effect on the Company’s results of operations because the full amount of the proposed settlement will be paid by the Company’s insurance carriers. As of April 1, 2015, the full amount of the proposed settlement was deposited by the insurance carriers into an escrow account. Though the Company believes the likelihood of approval of the settlement is probable, the Company cannot predict with certainty whether the settlement will be finally approved, and, if the settlement is not finally approved by the Court, the Company believes that it has meritorious defenses to the claims in the operative complaint.
	Other Legal and Regulatory Matters
	The Company owns subsidiaries including regulated entities that are subject to extensive oversight under federal, state and applicable international laws as well as self-regulatory organization ("SRO") rules. Changes in market structure and the need to remain competitive require constant changes to the Company's systems and order handling procedures. The Company makes these changes while continuously endeavoring to comply with many complex laws and rules. Compliance, surveillance and trading issues common in the securities industry are monitored by, reported to, and/or reviewed in the ordinary course of business by the Company's regulators in the U.S. and abroad. As a major order flow execution destination, the Company is named from time to time in, or is asked to respond to a number of regulatory matters brought by U.S. regulators, foreign regulators. SROs, as well actions brought by private plaintiffs, which arise from its business activities. There has recently been an increased focus by regulators on Anti-Money Laundering and sanctions compliance by broker-dealers and similar entities, as well as an enhanced interest on transactions involving microcap securities. The Company is currently the subject of various regulatory reviews and investigations by both U.S. and foreign regulators and SROs, including the SEC, FINRA, FCA and the AMF. In some instances, these matters may rise to a disciplinary action and/or a civil or administrative action.
	In addition, there has been an increased focus by Congress, federal and state regulators, the SROs and the media on market structure issues, and in particular, high frequency trading, ATS manner of operations, market fragmentation and complexity, colocation, access to market data feeds and remuneration arrangements, such as payment for order flow and exchange fee structures. The Company has received information requests from various authorities, including the SEC, requesting, among other items, information regarding these market structure matters, which the Company is in the process of responding.
	Lease and Contract Obligations
	Capital Leases
	The Company enters into capitalized lease obligations related to certain computer equipment. These obligations represent drawdowns under a revolving secured lending facility with a single lender. At March 31, 2015, the obligations have a weighted-average interest rate of 3.46% per annum and are on varying 3-year terms. The carrying amounts of the capital leases approximate fair value. The future minimum payments including interest under the capitalized leases at March 31, 2015 consist of (in thousands):


		Minimum Payments
	Nine months ended December 31, 2015	$	2,535

	2016	2,126

	2017	620

	Total	$	5,281

	The total interest expense related to capital leases for the three months ended March 31, 2015, and 2014 included in the Consolidated Statements Operations is as follows (in thousands):


		For the three months ended March 31,
		2015		2014
	Interest expense - Capital leases	$	68	$	88

	Operating Leases
	The Company leases office space under noncancelable operating leases. Certain office leases contain fixed dollar-based escalation clauses. Rental expense from continuing operations under the office leases was $4.6 million and $5.1 million for the three months ended March 31, 2015 and 2014, respectively, and is included in Occupancy and equipment rentals on the Consolidated Statements of Operations.
	The Company leases certain computer and other equipment under noncancelable operating leases. As of March 31, 2015, future minimum rental commitments under all noncancelable office, computer and equipment leases (“Gross Lease Obligations”), and Sublease Income were as follows (in thousands):


		Gross Lease		Sublease		Net Lease
		Obligations		Income		Obligations
	Nine months ending December 31, 2015	$	21,159	$	3,903	$	17,256

	Year ending December 31, 2016	27,934		4,994		22,940

	Year ending December 31, 2017	26,395		4,508		21,887

	Year ending December 31, 2018	25,550		4,211		21,339

	Year ending December 31, 2019	23,671		3,630		20,041

	Thereafter through December 31, 2027	64,589		10,244		54,345

	Total	$	189,298	$	31,490	$	157,808

	Contract Obligations
	During the normal course of business, the Company collateralizes certain leases or other contractual obligations through letters of credit or segregated funds held in escrow accounts. At March 31, 2015, the Company had provided letters of credit for $11.6 million, collateralized by cash, as a guarantee for several of its lease obligations and for a trading JV. In the ordinary course of business, KCG also has provided, and may provide in the future, unsecured guarantees with respect to the payment obligations of certain of its subsidiaries under trading, repurchase, financing and stock loan arrangements, as well as under certain leases. There were no compensation guarantees at March 31, 2015 or 2014 that extended beyond the respective year.

Financial_instruments_with_Off

Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk	Derivative Financial Instruments
	The Company enters into derivative transactions, primarily with respect to making markets in listed domestic options. In addition, the Company enters into derivatives to manage foreign currency exposure. Cash flows associated with such derivative activities are included in cash flows from operating activities on the Consolidated Statements of Cash Flows, when applicable.
	During the normal course of business, the Company enters into futures contracts. These financial instruments are subject to varying degrees of risks whereby the fair value of the securities underlying the financial instruments, may be in excess of, or less than, the contract amount. The Company is obligated to post collateral against certain futures contracts.
	The amounts and positions included in the tables below for futures contracts are classified as Level 1 while swaps and forward contracts are classified as Level 2 in the fair value hierarchy.
	The following tables summarize the fair value and number of derivative instruments held at March 31, 2015 and December 31, 2014 and the gains and losses included in the Consolidated Statements of Operations for the periods then ended. These instruments include those classified as Financial Instruments, owned at fair value, Financial instruments sold, not yet purchased at fair value, as well as futures contracts which are reported within Receivable from or Payable to brokers, dealers and clearing organizations in the Consolidated Statements of Financial Condition (fair value and gain (loss) in thousands):


				31-Mar-15
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	722			9,673			$	1,279		10,754

	Forward contracts	Financial instruments owned at fair value		—				—			234			1

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		2,250				2,208			894			3,429

	Swap contracts	Receivable from brokers, dealers and clearing organizations		46				1			—			—

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		87,412				261,752			78,427			256,523

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		4,409				6,896			8,061			11,399

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		21,535				12,459			22,332			12,050

	Total			$	116,374			292,989			$	111,227		294,156



				31-Dec-14
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	1,212			8,108			$	651		9,090

	Forward contracts	Financial instruments owned at fair value		60				1			—			—

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		1,790				2,590			2,047			3,085

	Swap contracts	Receivable from/Payable to brokers, dealers and clearing organizations		98				1			13			1

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		144,586				426,747			115,362			437,383

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		6,432				11,901			6,891			10,628

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		15,246				8,894			14,847			9,105

	Total			$	169,424			458,242			$	139,811		469,292



					Gain (Loss) Recognized
			Financial Statements		For the three months ended March 31,
			Location		2015				2014
	Derivative instruments not designated as hedging instruments:
	Foreign currency
	Futures contracts		Trading revenues, net		$	1,008			$	2,842

Forward contracts		Investment income and other, net		(294		)		432

Equity
Futures contracts		Trading revenues, net		2,377				8,737

Swap contracts		Trading revenues, net		329				1,282

Listed options (1)		Trading revenues, net		(13,550		)		(73,876		)
Fixed income
Futures contracts		Trading revenues, net		8,544				8,829

Commodity
Futures contracts		Trading revenues, net		15,435				13,570

				$	13,849			$	(38,184	)

Derivative instruments designated as hedging instruments:
Foreign exchange - forward contract		Accumulated other comprehensive income		$	—			$	168


(1)	Realized gains and losses on listed equity options relate to the Company’s market making activities in such options. Such market making activities also comprise trading in the underlying equity securities with gains and losses on such securities generally offsetting the gains and losses reported in this table. Gains and losses on such equity securities are also included in Trading revenues, net on the Company’s Consolidated Statements of Operations.
The Company has entered into and may continue to enter into International Swaps and Derivative Association, Inc (“ISDA”) master netting agreements with counterparties. Master agreements provide protection in bankruptcy in certain circumstances and, where legally enforceable, enable receivables and payables with the same counterparty to be settled or otherwise eliminated by applying amounts due against all or a portion of an amount due from the counterparty or a third party. The Company may also enter into bilateral trading agreements and other customer agreements that provide for the netting of receivables and payables with a given counterparty as a single net obligation.
Under the ISDA master netting agreements, the Company typically also executes credit support annexes, which provide for collateral, either in the form of cash or securities, to be posted or paid by a counterparty based on the fair value of the derivative receivable or payable based on the rates and parameters established in the credit support annex. In the event of counterparty’s default, provisions of the ISDA master agreement permit acceleration and termination of all outstanding transactions covered by the agreement such that a single amount is owed by, or to, the non-defaulting party. Any collateral posted can be applied to the net obligations, with any excess returned and the collateralized party has a right to liquidate the collateral. Any residual claim after netting is treated along with other unsecured claims in bankruptcy court.
The Company is also a party to clearing agreements with various central clearing parties. Under these arrangements, the central clearing counterparty facilitates settlement between counterparties based on the net payable owed or receivable due and, with respect to daily settlement, cash is generally only required to be deposited to the extent of the net amount. In the event of default, a net termination amount is determined based on the market values of all outstanding positions and the clearing organization or clearing member provides for the liquidation and settlement of the net termination amount among all counterparties to the open derivative contracts.
The table below provides information regarding (1) the extent to which, under enforceable master netting arrangements, such balances are presented net in the Consolidated Statements of Financial Condition as appropriate under GAAP and (2) the extent to which other rights of offset associated with these arrangements exist and could have had an effect on our financial position (in thousands):


	March 31, 2015	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition (3)				Net Amounts Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Available Collateral(1)		Counterparty Netting(2)

	Assets
	Listed options	$	87,412			$	—			$	87,412		$	—	$	—	$	87,412

	Swaps	46				—				46			—		—		46

	Futures	28,916				28,916				—			—		—		—

	Total Assets	$	116,374			$	28,916			$	87,458		$	—	$	—	$	87,458

	Liabilities
	Listed options	$	78,427			$	—			$	78,427		$	—	$	14,841	$	63,586

	Foreign currency forward contracts	234				—				234			—		—		234

	Futures	32,566				32,566				—			—		—		—

	Total Liabilities	$	111,227			$	32,566			$	78,661		$	—	$	14,841	$	63,820

(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
(2) Under master netting agreements with its counterparties, the Company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
(3) The full amount of the Liabilities related to Futures of $32.6 million has been netted against the Asset related to Futures of $28.9 million as the Company maintains margin in excess of the net liability and such margin is utilized to offset any excess liability.




	31-Dec-14	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Available Collateral(1)		Counterparty Netting(2)

	Assets
	Listed options	$	144,586			$	—			$	144,586		$	—	$	—	$	144,586

	Foreign currency forward contracts	60				—				60			—		—		60

	Swaps	98				13				85			—		—		85

	Futures	24,680				24,436				244			—		—		244

	Total Assets	$	169,424			$	24,449			$	144,975		$	—	$	—	$	144,975

	Liabilities
	Listed options	$	115,362			$	—			$	115,362		$	—	$	17,359	$	98,003

	Futures	24,436				24,436				—			—		—		—

	Swaps	13				13				—			—		—		—

	Total Liabilities	$	139,811			$	24,449			$	115,362		$	—	$	17,359	$	98,003

(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
(2) Under master netting agreements with its counterparties, the company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk
As a market maker in global equities, fixed income, futures, options, commodities and currencies, the majority of the Company’s securities transactions are conducted as principal or riskless principal with broker dealers and institutional counterparties primarily located in the United States. The Company self-clears substantially all of its U.S. equity and option securities transactions. The Company clears a portion of its securities transactions through third party clearing brokers. Foreign transactions are settled pursuant to global custody and clearing agreements with major U.S. banks. Substantially all of the Company’s credit exposures are concentrated with its clearing brokers, broker dealer and institutional counterparties. The Company’s policy is to monitor the credit standing of counterparties with which it conducts business.
In the normal course of its operations, the Company enters into contracts that contain a variety of representations and warranties which provide general indemnifications. The Company’s maximum exposure under these arrangements is unknown, as this would involve future claims that may be made against the Company that have not yet occurred. However, based on experience, the Company believes the risk of significant loss is minimal.
Financial instruments sold, not yet purchased, at fair value represent obligations to purchase such securities (or underlying securities) at a future date. The Company may incur a loss if the market value of the securities subsequently increases.
The Company currently has no loans outstanding to any former or current executive officer or director.

Business_Segments

Business Segments	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Business Segments	Business Segments
	As of March 31, 2015, the Company's operating segments comprised the following: (i) Market Making; (ii) Global Execution Services; and (iii) Corporate and Other.
	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, including trade executions conducted as an equities DMM on the NYSE and NYSE Amex. 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 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 AIM of the London Stock Exchange.
	The Global Execution Services segment comprises agency execution services and trading venues, offering trading in global equities, futures, options, and fixed income to institutions, banks and broker dealers. The Company generally earns commissions as an agent between principals for transactions that are executed within this segment; 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; (ii) institutional sales traders executing program, block and riskless principal trades in global equities and ETFs; (iii) a fixed income ECN that also offers trading applications; and (iv) an ATS for global equities.
	The Corporate and Other segment invests principally 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 contains functions that support the Company’s other segments.
	The Company’s revenues, income (loss) from continuing operations before income taxes (“Pre-tax earnings”) and total assets by segment are summarized in the following table (in thousands):


		Market		Global Execution Services		Corporate			Consolidated
		Making				and Other			Total
	For the three months ended March 31, 2015:
	Revenues	$	224,548	$	464,266	$	7,342		$	696,156

	Pre-tax earnings	39,340		381,058		(14,270		)	406,128

	Total assets	4,700,273		783,501		1,848,277			7,332,051

	For the three months ended March 31, 2014:
	Revenues	$	277,346	$	87,220	$	19,091		$	383,657

	Pre-tax earnings	76,032		2,016		(18,664		)	59,384

	Total assets	3,931,292		1,264,245		1,834,739			7,030,276

	In the first quarter of 2015, the Company began to allocate costs incurred to operate its self-clearing function to the Market Making and Global Execution Services segments and no longer report it as a distinct business within the Corporate and Other segment. Previously, these support costs were embedded within the internal clearing rates charged by the Corporate and Other segment to the various businesses, which eliminated during consolidation.
	Additionally, prior to 2015, funding costs of inventory positions were recorded in the Corporate and Other segment, primarily within Collateralized financing interest on the Consolidated Statements of Operations. These costs were subsequently charged out to the Market Making and Global Execution Services segments primarily through the Interest, net line item, with an equal and offsetting revenue item within the Corporate and Other segment. With the move of the self clearing team to a support function, these third party costs are now charged directly to the businesses within the Market Making and Global Execution Services segments. This shift in how the Company’s self clearing unit is reported has no impact to the Consolidated Statements of Operations, nor any of the individual line items within it. However, on a segment level, this decreases the amount of total revenues reported by the Corporate and Other segment, as it no longer records the offsetting revenue for these third party funding costs. This change in the measurement of segment profitability, which has no impact to the consolidated results, is reported prospectively, and, therefore, is not reflected in the financial results for any period prior to January 1, 2015.
	Included in Revenues and Pre-tax earnings within Global Execution Services for the three months ended March 31, 2015 are results of KCG Hotspot up through March 13, 2015, the date of the sale. Also included in these results is a gain related to the sale of KCG Hotspot of $385.0 million in Revenues and $373.8 million in Pre-tax earnings. Additionally, Revenue and Pre-tax earnings from KCG Hotspot and the Company's former FCM business are included in the Global Execution Services segment for the three months ended March 31, 2014.

Subsequent_Events

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events
	The Company has evaluated subsequent events through the date the consolidated financial statements were issued. The Company did not have any subsequent events requiring adjustment or disclosure in the consolidated financial statements except the following:
	Tender Offer
	On April 2, 2015, the Company’s Board of Directors authorized an expanded share repurchase program of up to $400.0 million of the Company’s Class A Common Stock and warrants (including the $55.0 million of remaining capacity under the previously authorized repurchase program). As part of this authority, the Company commenced, on May 4, 2015, a “modified Dutch auction” tender offer that will remain open for at least 20 business days. Under the terms of the tender offer, stockholders have the opportunity to sell the Company's Class A Common Stock to the Company at a specified price per share not less than $13.50 and not greater than $14.00. Upon expiration of the tender offer, and based on the number of shares tendered and the prices specified by the tendering stockholders, the Company will determine the lowest price within the range that will allow it to repurchase up to $330.0 million of the Company's Class A Common Stock (or all shares properly tendered and not properly withdrawn if the tender offer is not fully subscribed). All shares purchased by the Company in the tender offer will be purchased at the same price. If the aggregate purchase price for shares tendered at or below the specified purchase price exceeds $330.0 million, allocations will be made on a pro rata basis from stockholders tendering at or below the purchase price. Assuming the offer is fully subscribed, the Company will repurchase a minimum of 23.6 million shares, or 22 percent of its total shares outstanding excluding RSUs as of April 30, 2015. Assuming the tender offer is fully subscribed, approximately $70.0 million in authority will remain in the share repurchase program.
	The description above is for informational purposes only, is not a recommendation to buy or sell the Company's Class A Common Stock, and does not constitute an offer to buy or the solicitation to sell shares of KCG Class A Common Stock. The tender offer will be made only pursuant to the Offer to Purchase, Letter of Transmittal and related materials that KCG filed on May 4, 2015 with the SEC. Stockholders should read carefully the Offer to Purchase, Letter of Transmittal and related materials because they contain important information, including the various terms of, and conditions to, the tender offer. Stockholders may obtain a free copy of the tender offer statement on Schedule TO, the Offer to Purchase, Letter of Transmittal and other documents that the Company filed with the SEC at the SEC’s website at www.sec.gov or the investor information section of the Company’s website at www.kcg.com.
	Release of Escrow account
	As described in Footnote 11 "Debt", on March 13, 2015, the Company funded an escrow account maintained by Bank of New York with $330.2 million, which was an amount sufficient to redeem the 8.25% Senior Secured Notes in full and satisfy and discharge the 8.25% Senior Secured Notes Indenture.
	On April 13, 2015, the escrow amount of $330.2 million was released and the required amount was paid out to the 8.25% Senior Secured Notes holders and satisfied and discharged the Senior Secured Notes Indenture. As of April 13, 2015, the Company's Debt was approximately $495.0 million.
	With the release of the $330.2 million escrow account, charges for the make-whole premium of $16.5 million were recognized, and capitalized debt costs of $8.5 million associated with the 8.25% Senior Secured Notes were written off in April 2015.

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of consolidation and form of presentation	Basis of consolidation and form of presentation
	The Consolidated Financial Statements, prepared in conformity with generally accepted accounting principles ("GAAP"), include the accounts of the Company and its subsidiaries. All significant intercompany transactions and balances have been eliminated.
	Certain reclassifications have been made to the prior periods’ Consolidated Financial Statements in order to conform to the current period presentation. Such reclassifications are immaterial to both current and all previously issued financial statements taken as a whole and have no effect on previously reported Consolidated Net income.
Change in accounting principle	Change in accounting principle
	As discussed in Footnote 9 "Investments", as a result of the merger of BATS and Direct Edge Holdings LLC ("Direct Edge") in the first quarter of 2014, the Company changed its method of accounting for its investment in BATS from the cost method to the equity method.
Cash and cash equivalents	Cash and cash equivalents
	Cash and cash equivalents include money market accounts, which are payable on demand, and short-term investments with an original maturity of less than 90 days. The carrying amount of such cash equivalents approximates their fair value due to the short-term nature of these instruments. These assets would be categorized as Level 1 in the fair value hierarchy if they were required to be recorded at fair value.
Cash and cash equivalents segregated under federal and other regulations	Cash and cash equivalents segregated under federal and other regulations
	The Company maintains custody of customer funds and is obligated by rules and regulations mandated by the U.S. Securities and Exchange Commission (“SEC”) and the Commodity Futures Trading Commission (“CFTC”) to segregate or set aside cash and/or qualified securities to satisfy these regulations, which have been promulgated to protect customer assets. The amounts recognized as Cash and cash equivalents segregated under federal and other regulations approximate fair value.
Market making, sales, trading and execution activities	Market making, sales, trading and execution activities
	Financial instruments owned and Financial instruments sold, not yet purchased, relate to market making and trading activities, include listed and other equity securities, listed equity options and fixed income securities which are recorded on a trade date basis and carried at fair value. Trading revenues, net, which comprises trading gains, net of trading losses, are also recorded on a trade date basis.
	Commissions, which primarily includes commission equivalents earned on institutional client orders and related expenses are also recorded on a trade date basis. In 2014, commissions earned by the Company’s former FCM, which was sold in November 2014, were recorded net of any commissions paid to independent brokers and recognized on a half-turn basis.
	The Company’s third party clearing agreements call for payment or receipt of interest income, net of transaction-related interest charged by such clearing brokers, for facilitating the settlement and financing of securities transactions.
	Dividend income relating to financial instruments owned and dividend expense relating to financial instruments sold, not yet purchased, derived primarily from the Company’s market making activities are included as a component of Trading revenues, net on the Consolidated Statements of Operations.
Fair value of financial instruments	Fair value of financial instruments
	The Company values its financial instruments using a hierarchy of fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs when measuring fair value.
	The fair value hierarchy can be summarized as follows:

	•	Level 1—Valuations based on quoted prices in active markets for identical assets or liabilities that the Company has the ability to access. Since valuations are based on quoted prices that are readily and regularly available in an active market, valuation of these products does not entail a significant degree of judgment.

	•	Level 2—Valuations based on quoted prices in markets that are not active or for which all significant inputs are observable, either directly or indirectly.

	•	Level 3—Valuations based on inputs that are unobservable and significant to the overall fair value measurement.
	Changes in fair value are recognized in earnings each period for financial instruments that are carried at fair value.
Collateralized agreements and financings	Collateralized agreements and financings
	Collateralized agreements consist of securities borrowed and collateralized financings include securities loaned and financial instruments sold under agreements to repurchase.

	•	Securities borrowed and securities loaned transactions are recorded at the amount of cash collateral advanced or received. Securities borrowed transactions facilitate the securities settlement process and require the Company to deposit cash or other collateral with the lender. Securities loaned transactions help finance the Company’s securities inventory whereby the Company lends stock to counterparties in exchange for the receipt of cash or other collateral from the borrower. In these transactions, the Company receives or lends cash or other collateral in an amount generally in excess of the market value of the applicable securities borrowed or loaned. The Company monitors the market value of securities borrowed or loaned on a daily basis, and obtains additional collateral or refunds excess collateral as necessary.

	•	Financial instruments sold under agreements to repurchase are used to finance inventories of securities and other financial instruments and are recorded at their contractual amount. The Company has entered into bilateral and tri-party term and overnight repurchase agreements which bear interest at negotiated rates. The Company receives cash and makes delivery of financial instruments to a custodian who monitors the market value of these instruments on a daily basis. The market value of the instruments delivered must be equal to or in excess of the principal amount loaned under the repurchase agreements plus the agreed upon margin requirement. The custodian may request additional collateral, if appropriate.
Securitization activities	The Company’s securities borrowed, securities loaned and financial instruments sold under agreements to repurchase are recorded at amounts that approximate fair value. These items are recorded based upon their contractual terms and are not materially sensitive to shifts in interest rates because they are short-term in nature and are substantially collateralized pursuant to the terms of the underlying agreements. These items would be categorized as Level 2 in the fair value hierarchy if they were required to be recorded at fair value.
Investments	Investments
	Investments primarily comprise strategic noncontrolling equity ownership interests in financial services-related businesses and are held by the Company's non-broker dealer subsidiaries. These strategic investments are accounted for under the equity method, at cost or at fair value. The equity method of accounting is used when the Company has significant influence. Strategic investments are held at cost, less impairment if any, when the investment does not have a readily determined fair value, and the Company is not considered to exert significant influence on operating and financial policies of the investee. Strategic investments that are publicly traded are held at fair value and classified as available for sale securities on the Consolidated Statements of Financial Condition.
	Strategic investments are reviewed on an ongoing basis to ensure that the carrying values of the investments have not been impaired. If the Company determines that an impairment loss on a strategic investment has occurred due to a decline in fair value or other market conditions, the investment is written down to its estimated fair value.
	The Company maintains a non-qualified deferred compensation plan for certain employees and directors. This plan provides a return to the participants based upon the performance of various investments. In order to hedge its liability under this plan, the Company generally acquires the underlying investments and holds such investments until the deferred compensation liabilities are satisfied. Changes in value of such investments are recorded in Investment income and other, net, with a corresponding charge or credit to Employee compensation and benefits on the Consolidated Statements of Operations. Deferred compensation investments primarily consist of mutual funds, which are accounted for at fair value.
Goodwill and intangible assets	Goodwill and intangible assets
	The Company tests goodwill and intangible assets with an indefinite useful life for impairment annually or when an event occurs or circumstances change that signifies that the carrying amounts may not be recoverable. The Company capitalizes certain costs associated with the acquisition or development of internal-use software and amortizes the software over its estimated useful life of three years, commencing at the time the software is placed in service. The Company amortizes intangible assets with a finite life on a straight line basis over their estimated useful lives and tests for recoverability whenever events indicate that the carrying amounts may not be recoverable.
Payable to customers	Payable to customers
	Payable to customers primarily relate to amounts due on cash and margin transactions. Due to their short-term nature, such amounts approximate fair value.
Treasury stock	Treasury stock
	The Company records its purchases of treasury stock at cost as a separate component of stockholders’ equity. The Company may obtain treasury stock through purchases in the open market or through privately negotiated transactions. Certain treasury stock repurchases represent shares of KCG Class A Common Stock repurchased in satisfaction of tax withholding obligations upon vesting of restricted awards. The Company may re-issue treasury stock, at average cost, for the acquisition of new businesses and in certain other circumstances.
Foreign currency translation and foreign currency forward contracts	Foreign currency translation and foreign currency forward contracts
	The Company's foreign subsidiaries generally use the U.S. dollar as their functional currency. Effective January 1, 2014, one of the Company's U.K. subsidiaries changed its functional currency from British pounds to U.S. dollars. The Company has a subsidiary in India that utilizes the Indian Rupee as its functional currency.
	Assets and liabilities of the Indian subsidiary are translated at exchange rates at the end of a period. Revenues and expenses are translated at average exchange rates during the period. Gains and losses resulting from translating foreign currency financial statements into U.S. dollars are included in Accumulated other comprehensive income on the Consolidated Statements of Financial Condition and Cumulative translation adjustment, net of tax on the Consolidated Statements of Comprehensive Income.
	Gains or losses resulting from foreign currency transactions are included in Investment income and other, net on the Company’s Consolidated Statements of Operations. For the three months ended March 31, 2015 and 2014, the Company recorded losses of $0.2 million and $0.8 million, respectively on foreign currency transactions.
	The Company seeks to reduce the impact of fluctuations in foreign exchange rates on its net investment in certain non-U.S. operations through the use of foreign currency forward contracts. For foreign currency forward contracts designated as hedges, the Company assesses its risk management objectives and strategy, including identification of the hedging instrument, the hedged item and the risk exposure and how effectiveness is to be assessed prospectively and retrospectively. The effectiveness of the hedge is assessed based on the overall changes in the fair value of the forward contracts.
Stock and unit based compensation	Stock and unit based compensation
	Stock and unit based compensation is measured based on the grant date fair value of the awards. These costs are amortized over the requisite service period, which is typically the vesting period. Expected forfeitures are considered in determining stock-based employee compensation expense.
	The Company applies a non-substantive vesting period approach for stock-based awards related to KCG Class A Common Stock whereby the expense is accelerated for those employees and directors that receive options, stock appreciation rights ("SARs") and restricted stock units ("RSUs") and are eligible to retire prior to the vesting of such awards and in certain other circumstances.
Soft dollar expense	Soft dollar expense
	Under a commission management program, the Company allows institutional clients to allocate a portion of their gross commissions to pay for research and other services provided by third parties. As the Company acts as an agent in these transactions, it records such expenses on a net basis within Commissions and fees on the Consolidated Statements of Operations.
Depreciation, amortization and occupancy	Depreciation, amortization and occupancy
	Fixed assets are depreciated on a straight-line basis over their estimated useful lives of three to seven years. Leasehold improvements are being amortized on a straight-line basis over the shorter of the term of the related office lease or the expected useful life of the assets. The Company reviews fixed assets and leasehold improvements for impairment whenever events or changes in circumstances indicate that the related carrying amounts may not be recoverable.
	The Company recognizes rent expense under operating leases with fixed rent escalations, lease incentives and free rent periods on a straight-line basis over the lease term beginning on the date the Company takes possession of or controls the use of the space, including during free rent periods.
Lease loss accrual	Lease loss accrual
	The Company’s policy is to identify excess real estate capacity and where applicable, accrue for related future costs, net of projected sub-lease income upon the date the Company ceases to use the excess real estate. Such accrual is adjusted to the extent the actual terms of sub-leased property differ from the assumptions used in the calculation of the accrual.
Income taxes	Income taxes
	The Company is a corporation subject to U.S. corporate income tax as well as non-U.S. income taxes in the jurisdictions in which it operates. The Company records deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the financial reporting and tax bases of assets and liabilities and measures them using the enacted tax rates and laws that will be in effect when such differences are expected to reverse. The Company evaluates the recoverability of future tax deductions by assessing the adequacy of future expected taxable income from all sources, including reversal of temporary differences and forecasted operating earnings.
Variable interest entities	Variable interest entities
	A variable interest entity (“VIE”) is an entity that lacks one or more of the following characteristics (i) the total equity investment at risk is sufficient to enable the entity to finance its activities independently and (ii) the equity holders have the power to direct the activities of the entity that most significantly impact its economic performance, the obligation to absorb the losses of the entity and the right to receive the residual returns of the entity.
	The Company will be considered to have a controlling financial interest and will consolidate a VIE if it has both (i) the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance and (ii) the obligation to absorb losses of the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE.
	Effective January 2015, the Company owns 50% of the voting shares and 50% of the equity of a joint venture (“JV”) which maintains microwave communication networks in the U.S. and Europe. The Company and its JV partner each use the microwave networks in connection with their respective trading activities, and the JV sells excess bandwidth that is not utilized by the JV members to third parties. The Company pays the JV for the communication services that it uses, and such amounts are recorded within Communications and data processing on the Consolidated Statements of Operations.
	The Company does not have the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance; therefore it does not have a controlling financial interest in the JV and does not consolidate the JV. The Company records its interest in the JV under the equity method of accounting and records its investment in the JV within Investments and its amounts payable for communication services within Accrued expenses and other liabilities on the Consolidated Statements of Financial Condition. The Company records its pro-rata share of the JV’s earnings or losses within Investment income and other, net on the Consolidated Statements of Operations.
	The Company’s exposure to the obligations of this VIE is generally limited to its interests in the JV, which is the carrying value of the equity investment in the JV.
Use of estimates	Use of estimates
	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates.
Recently adopted accounting guidance	Recently adopted accounting guidance
	In April 2014, the Financial Accounting Standards Board (“FASB”) issued an Accounting Standard Update (“ASU”) that amends the requirements for reporting discontinued operations. Under the new guidance, discontinued operations reporting are limited to disposal transactions that represent strategic shifts having a major effect on operations and financial results. The amended guidance also enhances disclosures and requires assets and liabilities of a discontinued operation to be classified as such for all periods presented in the financial statements. The updated guidance is effective prospectively to all disposals occurring for interim and annual reporting periods after December 15, 2014, with early adoption permitted. The Company early adopted this ASU in 2014, which resulted in additional disclosures within the Company's Consolidated Financial Statements.
	In June 2014, the FASB issued an ASU that amends the accounting and disclosure guidance on repurchase agreements. The amended guidance requires entities to account for repurchase-to-maturity transactions as secured borrowings. Additional disclosures will be required for the nature of collateral pledged in repurchase agreements and similar transactions accounted for as secured borrowings. The accounting changes and additional disclosures about certain transferred financial assets accounted for as sales were effective for reporting periods beginning after December 15, 2014. The additional disclosures for securities financing transactions are required for annual reporting periods beginning after December 15, 2014 and for interim reporting periods beginning after March 15, 2015. Other than additional disclosure requirements, the adoption of this ASU did not have an impact on the Company’s Consolidated Financial Statements.
	Recent accounting guidance to be adopted in future periods
	In May 2014, the FASB issued an ASU that updates the principles for recognizing revenue. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The updated guidance is effective for interim and annual reporting periods beginning after December 15, 2016. Early adoption is not permitted. The Company is evaluating the impact of this ASU on its Consolidated Financial Statements.
	In June 2014, the FASB issued an ASU to resolve diverse accounting treatment for share based awards in which the terms of the award are related to a performance target that affects vesting. The ASU requires an entity to treat a performance target that could be achieved after the requisite service period as a performance condition. Additionally, compensation cost should be recognized in the period in which it becomes probable that the performance target will be achieved, and should represent the compensation cost attributable to the period(s) for which the requisite service has already been rendered; if the performance target becomes probable of being achieved before the end of the requisite service period, then the remaining unrecognized compensation cost should be recognized prospectively over the remaining requisite service period. The guidance is effective for reporting periods beginning after December 15, 2015 and may be applied prospectively or retrospectively. The Company does not expect adoption of this ASU to have an impact on the Company’s Consolidated Financial Statements.
	In August 2014, the FASB issued an ASU that requires an entity’s management to evaluate whether there are conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued. The guidance is effective for reporting periods beginning after December 15, 2016. Other than additional disclosure requirements, the adoption of this ASU is not expected to have an impact on the Company’s Consolidated Financial Statements.
	In February 2015, the FASB issued an ASU which requires entities to evaluate whether they should consolidate certain legal entities. The ASU simplifies consolidation accounting by reducing the number of consolidation models that an entity may apply. The guidance is effective for the Company beginning after December 15, 2015 and early adoption is permitted. The Company is evaluating the impact of this ASU on its Consolidated Financial Statements.
	In April 2015, the FASB issued an ASU regarding simplification of the presentation of debt issuance costs. The ASU requires that debt issuance costs related to a recognized debt liability be presented in the Consolidated Statement of Financial Condition as a direct deduction from the carrying amount of that debt liability. The guidance is effective for the Company retrospectively for fiscal years beginning after December 15, 2015 and early adoption is permitted. The Company is evaluating the impact of this ASU on its Consolidated Financial Statements.

Significant_Accounting_Policie2

Significant Accounting Policies (Tables)	3 Months Ended
Significant Accounting Policies (Tables)	Mar. 31, 2015
Accounting Policies [Abstract]
Schedule of Interest Income and Interest Expense	Interest income and interest expense which have been netted within Interest, net on the Consolidated Statements of Operations are as follows (in thousands):


		For the three months ended March 31,
		2015			2014
	Interest Income	$	3,771		$	3,351
	Interest Income	$	3,771		$	3,351
	Interest Expense	(3,794		)	(2,403		)
	Interest, net	$	(23	)	$	948
	Interest, net	$	(23	)	$	948
Net Trading Revenue Including Dividend Income and Expense	Trading revenues, net includes dividend income and expense as follows (in thousands):


		For the three months ended March 31,
		2015			2014
	Dividend Income	$	16,743		$	9,783
	Dividend Income	$	16,743		$	9,783
	Dividend Expense	$	(10,253	)	$	(7,575	)
Schedule of Variable Interest Entities	The following table presents the Company’s nonconsolidated VIE at March 31, 2015 (in thousands):


			Carrying Amount					Maximum Exposure to loss
			Asset			Liability		Maximum Exposure to loss		VIE's assets
	Equity investment		$	2,906		$	727	$	2,906	$	8,876
	Equity investment		$	2,906		$	727	$	2,906	$	8,876

Sales_of_Businesses_Tables

Sales of Businesses (Tables)	3 Months Ended
	Mar. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Summary of Balance Sheet and Results of Operations of Discontinued Operations	The KCG Hotspot assets and liabilities held for sale as of December 31, 2014 are summarized as follows (in thousands):


			December 31,
			2014
	Assets:
	Fixed assets, less accumulated depreciation		$	391

	Intangible assets, net accumulated amortization		34,696

	Other assets		5,397

	Total assets of business held for sale		$	40,484


	Liabilities:
	Accrued compensation expense		$	2,298

	Accrued expenses and other liabilities		58

	Total liabilities of business held for sale		$	2,356

	The revenues and results of operations of discontinued operations are summarized as follows (in thousands):


		For the three months ended March 31, 2014
	Additional Loss on Sale	$	(1,312	)

	Expenses:
	Compensation	$	46

	Other expenses	662

	Total Expenses	708

	Loss from discontinued operations	(2,020		)
	Income tax benefit	767

	Loss from discontinued operations, net of tax	$	(1,253	)

Assets_Segregated_or_Held_in_S1

Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations (Tables)	3 Months Ended
	Mar. 31, 2015
Regulated Operations [Abstract]
Cash and Securities Segregated Under U.S. Federal and Other Regulation	Cash and securities segregated under U.S. federal and other regulations relate to the Company’s regulated businesses and consist of the following (in thousands):


		March 31,		31-Dec-14
		2015
	Cash and cash equivalents segregated under federal or other regulations	$	3,000	$	3,361

	Total assets segregated or held in separate accounts under federal or other regulations	$	3,000	$	3,361

Fair_Value_Tables

Fair Value (Tables)	3 Months Ended
	Mar. 31, 2015
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following fair value hierarchy table presents information about the Company’s financial assets and liabilities measured at fair value (in thousands):


		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	March 31, 2015	Level 1			Level 2			Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,366,225		$	—		$	—		$	2,366,225

	Listed options	87,412			—			—			87,412

	U.S. government and Non-U.S. government obligations	118,563			—			—			118,563

	Corporate debt (2)	68,751			—			—			68,751

	Total Financial instruments owned, at fair value	2,640,951			—			—			2,640,951

	Investment in CME Group (3)	4,738			—			—			4,738

	Other investments(3)	—			1,569			—			1,569

	Total assets held at fair value	$	2,645,689		$	1,569		$	—		$	2,647,258

	Liabilities

	Financial instruments sold, not yet purchased, at fair value:
	Equities (1)	$	1,950,860		$	—		$	—		$	1,950,860

	Listed options	78,427			—			—			78,427

	U.S. government obligations	42,108			—			—			42,108

	Corporate debt (2)	70,655			—			—			70,655

	Foreign currency forward contracts	—			234			—			234

	Total liabilities held at fair value	$	2,142,050		$	234		$	—		$	2,142,284


	(1)	Equities of $696.3 million have been netted by their respective long and short positions by CUSIP number.

	(2)	Corporate debt securities of $16,000 have been netted by their respective long and short positions by CUSIP number

	(3)	Investment in CME Group and Other investments, which primarily consist of deferred compensation investments, are included within Investments on the Consolidated Statements of Financial Condition.



		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	December 31, 2014	Level 1			Level 2			Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,479,910		$	—		$	—		$	2,479,910

	Listed options	144,586			—			—			144,586

	U.S. government and Non-U.S. government obligations	22,983			—			—			22,983

	Corporate debt (2)	59,832			—			—			59,832

	Foreign currency forward contracts	—			60			—			60

	Total Financial instruments owned, at fair value	2,707,311			60			—			2,707,371

Investment in CME Group (3)	4,435			—			—			4,435

Other investments (3)	—			1,014			—			1,014

Total assets held at fair value	$	2,711,746		$	1,074		$	—		$	2,712,820

Liabilities
Financial instruments sold, not yet purchased, at fair value:
Equities (1)	$	2,069,342		$	—		$	—		$	2,069,342

Listed options	115,362			—			—			115,362

U.S. government obligations	18,953			—			—			18,953

Corporate debt (2)	82,050			—			—			82,050

Total liabilities held at fair value	$	2,285,707		$	—		$	—		$	2,285,707

(1) Equities of $743.1 million have been netted by their respective long and short positions by CUSIP number.
(2) Corporate debt instruments of $0.3 million have been netted by their respective long and short positions by CUSIP number.
(3) Investment in CME Group and Other investments, which primarily consist of deferred compensation investments, are included within Investments on the Consolidated Statements of Financial Condition.

Derivative_Financial_Instrumen1

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair Value of Derivative Instruments in Consolidated Statements of Financial Condition and Effect of Changes in Fair Value on Consolidated Statements of Operations	The following tables summarize the fair value and number of derivative instruments held at March 31, 2015 and December 31, 2014 and the gains and losses included in the Consolidated Statements of Operations for the periods then ended. These instruments include those classified as Financial Instruments, owned at fair value, Financial instruments sold, not yet purchased at fair value, as well as futures contracts which are reported within Receivable from or Payable to brokers, dealers and clearing organizations in the Consolidated Statements of Financial Condition (fair value and gain (loss) in thousands):


				31-Mar-15
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value		Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	722			9,673			$	1,279	10,754

	Forward contracts	Financial instruments owned at fair value		—				—			234		1

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		2,250				2,208			894		3,429

	Swap contracts	Receivable from brokers, dealers and clearing organizations		46				1			—		—

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		87,412				261,752			78,427		256,523

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		4,409				6,896			8,061		11,399

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		21,535				12,459			22,332		12,050

	Total			$	116,374			292,989			$	111,227	294,156



				31-Dec-14
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value		Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	1,212			8,108			$	651	9,090

	Forward contracts	Financial instruments owned at fair value		60				1			—		—

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		1,790				2,590			2,047		3,085

	Swap contracts	Receivable from/Payable to brokers, dealers and clearing organizations		98				1			13		1

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		144,586				426,747			115,362		437,383

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		6,432				11,901			6,891		10,628

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		15,246				8,894			14,847		9,105

	Total			$	169,424			458,242			$	139,811	469,292



					Gain (Loss) Recognized
			Financial Statements		For the three months ended March 31,
			Location		2015				2014
	Derivative instruments not designated as hedging instruments:
	Foreign currency
	Futures contracts		Trading revenues, net		$	1,008			$	2,842

	Forward contracts		Investment income and other, net		(294		)		432

	Equity
	Futures contracts		Trading revenues, net		2,377				8,737

Swap contracts		Trading revenues, net		329				1,282

Listed options (1)		Trading revenues, net		(13,550		)		(73,876		)
Fixed income
Futures contracts		Trading revenues, net		8,544				8,829

Commodity
Futures contracts		Trading revenues, net		15,435				13,570

				$	13,849			$	(38,184	)

Derivative instruments designated as hedging instruments:
Foreign exchange - forward contract		Accumulated other comprehensive income		$	—			$	168

Fair Value of Derivative Instruments Gain Loss Recognized

					Gain (Loss) Recognized
			Financial Statements		For the three months ended March 31,
			Location		2015				2014
	Derivative instruments not designated as hedging instruments:
	Foreign currency
	Futures contracts		Trading revenues, net		$	1,008			$	2,842

	Forward contracts		Investment income and other, net		(294		)		432

	Equity
	Futures contracts		Trading revenues, net		2,377				8,737

	Swap contracts		Trading revenues, net		329				1,282

	Listed options (1)		Trading revenues, net		(13,550		)		(73,876		)
	Fixed income
	Futures contracts		Trading revenues, net		8,544				8,829

	Commodity
	Futures contracts		Trading revenues, net		15,435				13,570

					$	13,849			$	(38,184	)

	Derivative instruments designated as hedging instruments:
	Foreign exchange - forward contract		Accumulated other comprehensive income		$	—			$	168


	(1)	Realized gains and losses on listed equity options relate to the Company’s market making activities in such options. Such market making activities also comprise trading in the underlying equity securities with gains and losses on such securities generally offsetting the gains and losses reported in this table. Gains and losses on such equity securities are also included in Trading revenues, net on the Company’s Consolidated Statements of Operations.
Derivative Instruments, Gain (Loss), Offsetting Table	The table below provides information regarding (1) the extent to which, under enforceable master netting arrangements, such balances are presented net in the Consolidated Statements of Financial Condition as appropriate under GAAP and (2) the extent to which other rights of offset associated with these arrangements exist and could have had an effect on our financial position (in thousands):


		March 31, 2015	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition (3)				Net Amounts Presented in the Statements of Financial Condition		Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Available Collateral(1)		Counterparty Netting(2)

		Assets
		Listed options	$	87,412			$	—			$	87,412	$	—	$	—	$	87,412

		Swaps	46				—				46		—		—		46

		Futures	28,916				28,916				—		—		—		—

		Total Assets	$	116,374			$	28,916			$	87,458	$	—	$	—	$	87,458

		Liabilities
		Listed options	$	78,427			$	—			$	78,427	$	—	$	14,841	$	63,586

		Foreign currency forward contracts	234				—				234		—		—		234

		Futures	32,566				32,566				—		—		—		—

		Total Liabilities	$	111,227			$	32,566			$	78,661	$	—	$	14,841	$	63,820

	(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
	(2) Under master netting agreements with its counterparties, the Company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
	(3) The full amount of the Liabilities related to Futures of $32.6 million has been netted against the Asset related to Futures of $28.9 million as the Company maintains margin in excess of the net liability and such margin is utilized to offset any excess liability.




		31-Dec-14	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts Presented in the Statements of Financial Condition		Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Available Collateral(1)		Counterparty Netting(2)

		Assets
		Listed options	$	144,586			$	—			$	144,586	$	—	$	—	$	144,586

		Foreign currency forward contracts	60				—				60		—		—		60

		Swaps	98				13				85		—		—		85

		Futures	24,680				24,436				244		—		—		244

		Total Assets	$	169,424			$	24,449			$	144,975	$	—	$	—	$	144,975

		Liabilities
		Listed options	$	115,362			$	—			$	115,362	$	—	$	17,359	$	98,003

		Futures	24,436				24,436				—		—		—		—

		Swaps	13				13				—		—		—		—

		Total Liabilities	$	139,811			$	24,449			$	115,362	$	—	$	17,359	$	98,003

	(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
	(2) Under master netting agreements with its counterparties, the company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.

Collateralized_Transactions_Ta

Collateralized Transactions (Tables)	3 Months Ended
	Mar. 31, 2015
Collateralized Agreements [Abstract]
Financial Instruments at Fair Value Received as Collateral that were Permitted to be Delivered or Repledged	The table below presents financial instruments at fair value received as collateral and included within Securities borrowed or Receivable from brokers, dealers and clearing organizations on the Consolidated Statements of Financial Condition that were permitted to be delivered or repledged and that were delivered or repledged by the Company as well as the fair value of financial instruments which could be further repledged by the receiving party (in thousands):


		March 31,			December 31,
		2015			2014
	Collateral permitted to be delivered or repledged	$	1,614,177		$	1,586,700

	Collateral that was delivered or repledged	1,508,355			1,485,267

	Collateral permitted to be further repledged by the receiving counterparty	239,757			147,696

Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge	The table below presents information about assets pledged by the Company (in thousands):


		March 31,			December 31,
		2015			2014
	Financial instruments owned, at fair value, pledged to counterparties that have the right to deliver or repledge	$	506,532		$	536,124

	Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	951,587			979,652

Assets and Liabilities Subject to Netting	The gross amounts of assets and liabilities subject to netting and gross amounts offset in the Consolidated Statements of Financial Condition were as follows (in thousands):


		March 31, 2015	Gross Amounts Recognized			Gross Amounts Offset in the Statements of Financial Condition			Net Amounts Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition					Net Amount
												Available Collateral(1)			Counterparty Netting(2)

		Assets
		Securities borrowed	$	1,685,850		$	—		$	1,685,850		$	1,612,086		$	13,355	$	60,409

		Receivable from brokers, dealers and clearing organizations (3)	133,935			—			133,935			133,829			—		106

		Total Assets	$	1,819,785		$	—		$	1,819,785		$	1,745,915		$	13,355	$	60,515

		Liabilities
		Securities loaned	$	792,887		$	—		$	792,887		$	760,775		$	13,355	$	18,757

		Financial instruments sold under agreements to repurchase	905,567			—			905,567			905,508			—		59

		Total Liabilities	$	1,698,454		$	—		$	1,698,454		$	1,666,283		$	13,355	$	18,816

	(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
	(2) Under master netting agreements with its counterparties, the Company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
	(3) Represents financial instruments repurchased under agreement to resell.


		31-Dec-14	Gross Amounts Recognized			Gross Amounts Offset in the Statements of Financial Condition			Net Amounts Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition					Net Amount
												Available Collateral(1)			Counterparty Netting(2)

		Assets
		Securities borrowed	$	1,632,062		$	—		$	1,632,062		$	1,570,194		$	15,782	$	46,086

		Receivable from brokers, dealers and clearing organizations (3)	21,545			—			21,545			21,425			—		120

		Total Assets	$	1,653,607		$	—		$	1,653,607		$	1,591,619		$	15,782	$	46,206

		Liabilities
		Securities loaned	$	707,744		$	—		$	707,744		$	682,389		$	15,782	$	9,573

		Financial instruments sold under agreements to repurchase	933,576			—			933,576			933,560			—		16

		Total Liabilities	$	1,641,320		$	—		$	1,641,320		$	1,615,949		$	15,782	$	9,589

	(1) Includes securities received or delivered under collateral arrangements with counterparties that could be liquidated in the event of a counterparty default and thus offset against a counterparty's rights and obligations under the respective repurchase agreements or securities borrowing or lending arrangements.
	(2) Under master netting agreements with its counterparties, the company has the legal right of offset with a counterparty, which incorporates all of the counterparty's outstanding rights and obligations under the arrangement. These balances reflect additional credit risk mitigation that is available by counterparty in the event of a counterparty's default, but which are not netted in the Statement of Financial Condition because other netting provisions under U.S. GAAP are not met.
	(3) Represents financial instruments purchased under agreement to resell.
Maturities of Assets Sold Under Repurchase Agreements	Maturities of Securities loaned and Financial instruments sold under agreements to repurchase are provided in the table below (dollars in thousands):


	As of March 31, 2015	Overnight			0 - 30 days			31 - 60 days			61 - 90 days			Total
	Securities loaned	$	792,887		$	—		$	—		$	—		$	792,887

	Financial instruments sold under agreements to repurchase	85,567			445,000			225,000			150,000			905,567

	Total	$	878,454		$	445,000		$	225,000		$	150,000		$	1,698,454



	As of December 31, 2014	Overnight			0 - 30 days			31 - 60 days			61 - 90 days			Total
	Securities loaned	$	707,744		$	—		$	—		$	—		$	707,744

	Financial instruments sold under agreements to repurchase	73,576			410,000			325,000			125,000			933,576

	Total	$	781,320		$	410,000		$	325,000		$	125,000		$	1,641,320

Receivable_from_and_Payable_to1

Receivable from and Payable to Brokers, Dealers and Clearing Organizations (Tables)	3 Months Ended
	Mar. 31, 2015
Brokers and Dealers [Abstract]
Schedule of Amounts Receivable from and Payable to Brokers, Dealers and Clearing Organizations	Amounts receivable from and payable to brokers, dealers and clearing organizations consist of the following (in thousands):


		March 31,		December 31, 2014
		2015
	Receivable:
	Clearing organizations and other	$	622,225	$	1,073,480

	Financial instruments purchased under agreement to resell	133,935		21,545

	Securities failed to deliver	162,346		93,808

	Total Receivable	$	918,506	$	1,188,833

	Payable:
	Clearing organizations and other	$	485,866	$	350,627

	Securities failed to receive	52,977		325,462

	Total Payable	$	538,843	$	676,089

Investments_Tables

Investments (Tables)	3 Months Ended
	Mar. 31, 2015
Investments [Abstract]
Summary of Investments	Investments primarily comprise strategic investments and deferred compensation investments. Investments consist of the following (in thousands):


		March 31,		December 31,
		2015		2014
	Strategic investments:
	Investments accounted for under the equity method	$	90,369	$	86,328

	Investments held at fair value	4,738		4,435

	Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	8,948		8,949

	Total Strategic investments	104,055		99,712

	Other investments	1,569		1,014

	Total Investments	$	105,624	$	100,726

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Goodwill	The following table summarizes the Company’s goodwill by segment (in thousands):


		March 31,				December 31, 2014
		2015
	Market Making	$	16,404			$	16,404

	Global Execution Services	907				907

	Total	$	17,311			$	17,311

Finite-Lived Intangible Assets, Net of Accumulated Amortization	The following tables summarize the Company’s Intangible assets, net of accumulated amortization by segment and type (in thousands):


		March 31,				December 31, 2014
		2015
	Market Making
	Technology	$	48,090			$	50,542

	Trading rights	43,191				44,358

	Total	91,281				94,900

	Global Execution Services (1)
	Technology	16,900				18,200

	Customer relationships	10,472				10,833

	Trade names	825				850

	Total	28,197				29,883

	Corporate and Other
	Technology	9,750				10,500

	Total	$	129,228			$	135,283


	(1)	Excluded from the December 31, 2014 balance is $34.7 million of intangibles related to the KCG Hotspot which was held for sale. As noted in Footnote 3 "Sales of businesses", such amount is included in Assets of business held for sale at December 31, 2014.


			March 31,				December 31, 2014
			2015
	Technology (1)	Gross carrying amount	$	117,729			$	115,804

		Accumulated amortization	(42,989		)		(36,562		)
		Net carrying amount	74,740				79,242

	Trading rights (2)	Gross carrying amount	62,468				62,468

		Accumulated amortization	(19,277		)		(18,110		)
		Net carrying amount	43,191				44,358

	Customer relationships (3)	Gross carrying amount	13,000				13,000

		Accumulated amortization	(2,528		)		(2,167		)
		Net carrying amount	10,472				10,833

	Trade names (4)	Gross carrying amount	1,000				1,000

		Accumulated amortization	(175		)		(150		)
		Net carrying amount	825				850

	Total	Gross carrying amount	194,197				192,272

		Accumulated amortization	(64,969		)		(56,989		)
		Net carrying amount	$	129,228			$	135,283


	(1)	The weighted average remaining life for technology, including capitalized software, was approximately 3 years for both March 31, 2015 and December 31, 2014. Excluded from the December 31, 2014 balance is $13.1 million of technology assets related to KCG Hotspot which as noted in Footnote 3 "Sales of businesses", is included in Assets of business held for sale at December 31, 2014.

	(2)	Trading rights provide the Company with the rights to trade on certain exchanges. The weighted average remaining life of trading rights with definite useful lives was approximately 7 years for both March 31, 2015 and December 31, 2014. As of March 31, 2015 and December 31, 2014, $6.8 million of trading rights had indefinite useful lives.

	(3)	Customer relationships relate to KCG BondPoint. The weighted average remaining life was approximately 7 and 8 years as of March 31, 2015 and December 31, 2014, respectively. Lives may be reduced depending upon actual retention rates. Excluded from the December 31, 2014 balance is $19.0 million of customer relationships related to KCG Hotspot which as noted in Footnote 3 "Sales of businesses", is included in Assets of business held for sale at December 31, 2014.

(4)	Trade names relate to KCG BondPoint. The weighted average remaining life was approximately 8 and 9 years as of March 31, 2015 and December 31, 2014, respectively. Excluded from the December 31, 2014 balance is $2.6 million of the trade name related to KCG Hotspot which as noted in Footnote 3 "Sales of businesses", is included in Assets of business held for sale at December 31, 2014.
Summary of Amortization Expense Relating to Intangible Assets	The following table summarizes the Company’s amortization expense from continuing operations relating to Intangible assets (in thousands):


		For the three months ended March 31,
		2015				2014
	Amortization expense	$	9,249			$	8,494

Summary of Estimated Amortization Expense for Future Years	As of March 31, 2015, the following table summarizes the Company’s estimated amortization expense for future periods (in thousands):


		Amortization
		expense
	For the nine months ended December 31, 2015	$	23,671

	For the year ended December 31, 2016	30,679

	For the year ended December 31, 2017	27,892

	For the year ended December 31, 2018	15,788

	For the year ended December 31, 2019	6,211

Debt_Tables

Debt (Tables)	3 Months Ended
Debt (Tables)	Mar. 31, 2015
Debt Disclosure [Abstract]
Summary of Long-term Debt	The carrying value and fair value of the Company's debt is as follows (in thousands):


		March 31, 2015					December 31, 2014
		Carrying Amount			Fair Value		Carrying Amount		Fair Value
	Cash Convertible Senior Subordinated Notes	$	—		$	—	$	117,259	$	116,819
	Cash Convertible Senior Subordinated Notes	$	—		$	—	$	117,259	$	116,819
	8.25% Senior Secured Notes	305,000			329,747		305,000		309,194
	8.25% Senior Secured Notes	305,000			329,747		305,000		309,194
	6.875% Senior Secured Notes	494,847			484,950		—		—
	6.875% Senior Secured Notes	494,847			484,950		—		—
	Total Debt	$	799,847		$	814,697	$	422,259	$	426,013
	Total Debt	$	799,847		$	814,697	$	422,259	$	426,013
Debt Instrument Redemption

	Year		Percentage
	2017		103.438	%
	2018		101.719	%
	2019 and thereafter		100	%
Recorded Expenses with Respect to Long-term Debt	The Company recorded expenses with respect to the Debt as follows (in thousands):


		For the three months ended March 31,
		2015			2014
	Interest expense	$	8,158		$	9,416
	Interest expense	$	8,158		$	9,416
	Amortization of debt issuance cost (1)	934			8,766
	Amortization of debt issuance cost (1)	934			8,766
	Commitment fee	394			397
	Commitment fee	394			397
	Total	$	9,486		$	18,579
	Total	$	9,486		$	18,579

	(1)	For the three months ended March 31, 2014, $7.6 million of amortization of debt issuance cost incurred is included in Writedown of debt issuance costs while $1.6 million which includes the remaining amortization of the debt issuance costs and the commitment fee is included in Other expenses.

Related_Parties_Tables

Related Parties (Tables)	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
Summary of Balances and Transactions with Related Parties or Their Affiliates	As of the date and period indicated below, the Company had the following balances and transactions with the related parties or their affiliates (in thousands):


		For the three months ended March 31,
	Statements of Operations	2015			2014
	Revenues
	Commissions and fees	$	3,492		$	5,146

	Trading revenues, net	2,101			814

	Interest, net	226			188

	Total revenues from related parties	$	5,819		$	6,148

	Expenses
	Execution and clearance fees(1)	$	(4,628	)	$	(4,011	)
	Communications and data processing	1,110			—

	Payment for order flow	1,180			—

	Collateralized financing interest	113			159

	Professional fees	5,507			—

	Other expense	621			400

	Total expenses incurred with respect to related parties	$	3,903		$	(3,452	)

	(1) Represents net volume based fees received from providing liquidity to related trading venues.


	Statements of Financial Condition	March 31,			December 31,
		2015			2014
	Assets
	Securities borrowed	$	41,600		$	26,110

	Receivable from brokers, dealers and clearing organizations	92,739			20,075

	Other assets	12,247			—

	Liabilities
	Securities loaned	$	3,543		$	7,376

	Payable to brokers, dealers and clearing organizations	64,757			8,509

	Accrued expenses and other liabilities	4,693			5,667

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Compensation Expense Relating to Restricted Awards	Compensation expense from continuing operations relating to RSUs, which is primarily recorded in Employee compensation and benefits, and the corresponding income tax benefit, which is recorded in Income tax expense on the Consolidated Statements of Operations are presented in the following table (in thousands):


		For the three months ended March 31,
		2015			2014
	Stock award compensation expense	$	14,255		$	16,238

	Income tax benefit	5,417			6,170

	The following is a summary of the changes in the incentive units for the three months ended March 31, 2015 (units in thousands):


		Vested
	Incentive units at December 31, 2014	38

	Issued	—

	Vested	—

	Exercised	—

	Canceled	—

	Incentive units at March 31, 2015	38

Summary of Restricted Awards Activity	The following table summarizes restricted awards activity for the three months ended March 31, 2015 (awards in thousands):


			Restricted Stock Units
			Number of		Weighted-
			Units		Average
					Grant date
					Fair Value
	Outstanding at December 31, 2014		9,147		$	10.77

	Granted		2,385		12.22

	Vested		(2,055	)	11.04

	Forfeited		(322	)	11.25

	Outstanding at March 31, 2015		9,155		$	11.07

Compensation Expense Relating to Stock Options	Compensation expense from continuing operations relating to stock options and SARs, all of which was recorded in Employee compensation and benefits, as well as the corresponding income tax benefit, which is recorded in Income tax expense on the Consolidated Statements of Operations are as follows (in thousands):


		For the three months ended March 31,
		2015			2014
	Stock option and SAR compensation expense	$	1,013		$	961

	Income tax benefit	385			365

Summary of Stock Option Activity	The following table summarizes stock option and SAR activity and stock options exercisable for the three months ended March 31, 2015 (awards in thousands):


			Number of Stock Awards		Weighted-			Aggregate		Weighted-
					Average			Intrinsic		Average
					Exercise			Value		Remaining
					Price					Life (years)
	Outstanding at December 31, 2014 (1)		4,691		$	17.41

	Granted at market value		—		—

	Exercised		—		—

	Forfeited or expired		(84	)	17.1

	Outstanding at March 31, 2015 (1)		4,607		$	17.42		$	4,687	3.15

	Exercisable at March 31, 2015		1,782		$	20.72		$	1,640	2.95

	Available for future grants at March 31, 2015 (2)		15,086

	(1) Includes 1.7 million of SARs.
	(2) Represents options, SARs and awards available for grant under the KCG Plan.
Schedule of Compensation Costs	Compensation expense (benefit) related to the Incentive units which are recorded within Employee compensation and benefits on the Consolidated Statements of Operations are as follows (in thousands):


		For the three months ended March 31,
		2015			2014
	Incentive units	$	153		$	(174	)

Income_Taxes_Tables

Income Taxes (Tables)	3 Months Ended
Income Taxes (Tables)	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Tax Rate Reconciliation	The following table reconciles the U.S. federal statutory income tax rate to the Company's actual income tax rate from continuing operations (in thousands):


		For the three months ended March 31,
		2015		2014

	U.S. federal statutory income tax rate	35	%	35	%
	U.S. state and local income taxes, net of U.S.
	Federal income tax effect	3.6	%	2.2	%
	Nondeductible expenses(1)	0	%	0.3	%
	Foreign Taxes	0	%	0.1	%
	Other, net	0	%	0.2	%
	Actual income tax rate	38.6	%	37.8	%
	(1) Nondeductible expenses include nondeductible compensation and meals and entertainment

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Accumulated Other Comprehensive Income	The following table presents changes in Accumulated other comprehensive income, net of tax by component for the three months ended March 31, 2015 and 2014 (in thousands):


		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments			Total
	Balance December 31, 2014	$	352		$	1,781		$	2,133

	Other comprehensive income (loss)	188			(623		)	(435		)

	Balance March 31, 2015	$	540		$	1,158		$	1,698



		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments			Total
	Balance December 31, 2013	$	36		$	1,365		$	1,401

	Other comprehensive income (loss)	(137		)	201			64

	Balance March 31, 2014	$	(101	)	$	1,566		$	1,465

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	3 Months Ended
	Mar. 31, 2015
Earnings Per Share [Abstract]
Schedule of Reconciliation of Earnings Per Share	The following is a reconciliation of the numerators and denominators of the basic and diluted earnings per share computations from continuing operations for the three months ended March 31, 2015 and 2014 (in thousands):


		For the three months ended March 31,
		2015				2014
		Numerator /		Denominator /		Numerator /		Denominator /
		net income		shares		net income		shares
	Income from continuing operations and shares used in basic calculations	$	249,301	110,782		$	36,917	115,569

	Effect of dilutive stock based awards
	Restricted awards			2,410				2,233

	Stock options and SARs			286				96

	Warrants			137				—

	Income from continuing operations and shares used in diluted calculations	$	249,301	113,615		$	36,917	117,898

	Basic earnings per common share from continuing operations			$	2.25			$	0.32

	Diluted earnings per common share from continuing operations			$	2.19			$	0.31

Commitments_and_Contingent_Lia1

Commitments and Contingent Liabilities (Tables)	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Lease and Contract Obligations	The future minimum payments including interest under the capitalized leases at March 31, 2015 consist of (in thousands):


		Minimum Payments
	Nine months ended December 31, 2015	$	2,535

	2016	2,126

	2017	620

	Total	$	5,281

Schedule of Interest Expense, Capital Lease	The total interest expense related to capital leases for the three months ended March 31, 2015, and 2014 included in the Consolidated Statements Operations is as follows (in thousands):


		For the three months ended March 31,
		2015		2014
	Interest expense - Capital leases	$	68	$	88

Schedule of Future Minimum Rental Payments for Operating Leases	As of March 31, 2015, future minimum rental commitments under all noncancelable office, computer and equipment leases (“Gross Lease Obligations”), and Sublease Income were as follows (in thousands):


		Gross Lease		Sublease		Net Lease
		Obligations		Income		Obligations
	Nine months ending December 31, 2015	$	21,159	$	3,903	$	17,256

	Year ending December 31, 2016	27,934		4,994		22,940

	Year ending December 31, 2017	26,395		4,508		21,887

	Year ending December 31, 2018	25,550		4,211		21,339

	Year ending December 31, 2019	23,671		3,630		20,041

	Thereafter through December 31, 2027	64,589		10,244		54,345

	Total	$	189,298	$	31,490	$	157,808

Business_Segments_Tables

Business Segments (Tables)	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Income (Loss) from Continuing Operations Before Income Taxes and Total Assets by Segment	The Company’s revenues, income (loss) from continuing operations before income taxes (“Pre-tax earnings”) and total assets by segment are summarized in the following table (in thousands):


		Market		Global Execution Services		Corporate			Consolidated
		Making				and Other			Total
	For the three months ended March 31, 2015:
	Revenues	$	224,548	$	464,266	$	7,342		$	696,156

	Pre-tax earnings	39,340		381,058		(14,270		)	406,128

	Total assets	4,700,273		783,501		1,848,277			7,332,051

	For the three months ended March 31, 2014:
	Revenues	$	277,346	$	87,220	$	19,091		$	383,657

	Pre-tax earnings	76,032		2,016		(18,664		)	59,384

	Total assets	3,931,292		1,264,245		1,834,739			7,030,276

Significant_Accounting_Policie3

Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Change in accounting principle
Investment income and other, net	$387,423,000	$12,155,000
Cash and cash equivalents
Maximum maturity of short-term investments	90 days
VIE [Member]
Variable Interest Entity, Not Primary Beneficiary, Disclosures [Abstract]
Ownership percentage	50.00%
Minimum [Member]
Goodwill and intangible assets
Amortization period	1 year
Foreign currency translation and foreign currency forward contracts
Fixed assets, useful life	3 years
Maximum [Member]
Goodwill and intangible assets
Amortization period	8 years
Foreign currency translation and foreign currency forward contracts
Fixed assets, useful life	7 years
Foreign Currency Transactions [Member]
Foreign currency translation and foreign currency forward contracts
Investment gains (losses)	-200,000	-800,000
Software [Member]
Goodwill and intangible assets
Amortization period	3 years
BATS and DE [Member]
Change in accounting principle
Merger related transaction costs		6,600,000
BATS and DE [Member] \| Investment Income and Other, Net [Member]
Change in accounting principle
Investment income and other, net		9,600,000
DE [Member]
Change in accounting principle
Investment income and other, net		$16,200,000

Significant_Accounting_Policie4

Significant Accounting Policies - Schedule of Interest Income and Interest Expense (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accounting Policies [Abstract]
Interest Income	$3,771	$3,351
Interest Expense	-3,794	-2,403
Interest, net	($23)	$948

Significant_Accounting_Policie5

Significant Accounting Policies - Net Trading Revenue Including Dividend Income and Expense (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accounting Policies [Abstract]
Dividend Income	$16,743	$9,783
Dividend Expense	($10,253)	($7,575)

Significant_Accounting_Policie6

Significant Accounting Policies - Nonconsolidated VIEs (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Variable Interest Entity [Line Items]
Carrying Amount, Asset	$2,906
Carrying Amount, Liability	727
Maximum Exposure to loss	2,906
VIE's assets	7,332,051	6,830,654	7,030,276
VIE [Member]
Variable Interest Entity [Line Items]
VIE's assets	$8,876

Sales_of_Businesses_Income_Dis

Sales of Businesses - Income Disclosures (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Expenses:
Compensation	$106,718	$122,319
Other expense	7,808	8,643
Total expenses	290,028	324,273
Loss from discontinued operations, net of tax	0	-1,253
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Additional Loss on Sale		-1,312
Expenses:
Compensation		46
Other expense		662
Total expenses		708
Loss from discontinued operations		-2,020
Income tax benefit		767
Loss from discontinued operations, net of tax		($1,253)

Sales_of_Businesses_Balance_Sh

Sales of Businesses - Balance Sheet Disclosures (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Liabilities:
Total liabilities of business held for sale	$0	$2,356
KCG Hotspot [Member]
Assets:
Fixed assets, less accumulated depreciation		391
Intangible assets, net accumulated amortization		34,696
Other assets		5,397
Total assets of business held for sale		40,484
Liabilities:
Accrued compensation expense		2,298
Accrued expenses and other liabilities		58
Total liabilities of business held for sale		$2,356

Sales_of_Businesses_Additional

Sales of Businesses - Additional Information (Details) (USD $)	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Professional fees	$11,181,000	$5,402,000
KCG Hotspot [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds related to tax sharing arrangement			70,000,000
Sale period			3 years
Gain from sale of business	373,800,000
Professional fees	6,700,000
Compensation costs	4,500,000
KCG Hotspot [Member] \| Other Assets [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Additional consideration, fair value	62,100,000		62,100,000
KCG Hotspot [Member] \| Investment Income and Other, Net [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from sale of business			$385,000,000

Assets_Segregated_or_Held_in_S2

Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations - (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Regulated Operations [Abstract]
Cash and cash equivalents segregated under federal or other regulations	$3,000	$3,361
Total assets segregated or held in separate accounts under federal or other regulations	$3,000	$3,361

Fair_Value_Financial_Assets_an

Fair Value - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	$2,640,951	$2,707,371
Financial instrument assets	2,647,258	2,712,820
Financial instruments sold, not yet purchased, at fair value	2,142,284	2,285,707
Total fair value of financial instrument liabilities	2,142,284	2,285,707
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,366,225	2,479,910
Financial instruments sold, not yet purchased, at fair value	1,950,860	2,069,342
Securities netted by respective long and short positions	696,300	743,100
Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	87,412	144,586
Financial instruments sold, not yet purchased, at fair value	78,427	115,362
US Government and Non-US Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	118,563	22,983
Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	68,751	59,832
Financial instruments sold, not yet purchased, at fair value	70,655	82,050
Securities netted by respective long and short positions	16	300
Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	4,738	4,435
Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	1,569	1,014
U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	42,108	18,953
Foreign Currency Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value		60
Financial instruments sold, not yet purchased, at fair value	234
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,640,951	2,707,311
Financial instrument assets	2,645,689	2,711,746
Total fair value of financial instrument liabilities	2,142,050	2,285,707
Level 1 [Member] \| Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,366,225	2,479,910
Financial instruments sold, not yet purchased, at fair value	1,950,860	2,069,342
Level 1 [Member] \| Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	87,412	144,586
Financial instruments sold, not yet purchased, at fair value	78,427	115,362
Level 1 [Member] \| US Government and Non-US Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	118,563	22,983
Level 1 [Member] \| Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	68,751	59,832
Financial instruments sold, not yet purchased, at fair value	70,655	82,050
Level 1 [Member] \| Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	4,738	4,435
Level 1 [Member] \| Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	0	0
Level 1 [Member] \| U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	42,108	18,953
Level 1 [Member] \| Foreign Currency Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value		0
Financial instruments sold, not yet purchased, at fair value	0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	60
Financial instrument assets	1,569	1,074
Total fair value of financial instrument liabilities	234	0
Level 2 [Member] \| Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	0
Financial instruments sold, not yet purchased, at fair value	0	0
Level 2 [Member] \| Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	0
Financial instruments sold, not yet purchased, at fair value	0	0
Level 2 [Member] \| US Government and Non-US Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	0
Level 2 [Member] \| Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	0	0
Financial instruments sold, not yet purchased, at fair value	0	0
Level 2 [Member] \| Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	0	0
Level 2 [Member] \| Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	1,569	1,014
Level 2 [Member] \| U.S. Government Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value	0	0
Level 2 [Member] \| Foreign Currency Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value		60
Financial instruments sold, not yet purchased, at fair value	$234

Fair_Value_Additional_Informat

Fair Value - Additional Information (Detail)	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014
Fair Value - Additional Information (Detail)	USD ($)	INR	USD ($)	INR
Fair Value Disclosures [Abstract]
Transfers of financial instruments between levels	$0		$0
Transfers of financial instruments between level 2 to level 1	0		0
Notional value of foreign currency forward	$10,900,000	700,000,000	$10,900,000	700,000,000

Derivative_Financial_Instrumen2

Derivative Financial Instruments - Fair Value of Derivative Instruments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	contract	contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	$116,374	$169,424
Assets, Contract	292,989	458,242
Liabilities, Fair Value	111,227	139,811
Liabilities, Contract	294,156	469,292
Foreign Currency Futures Contracts [Member] \| Receivable from Brokers, Dealers and Clearing Organizations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	722	1,212
Assets, Contract	9,673	8,108
Liabilities, Fair Value	1,279	651
Liabilities, Contract	10,754	9,090
Foreign Currency Forward Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value		60
Liabilities, Fair Value	234
Foreign Currency Forward Contracts [Member] \| Financial Instruments Owned, at Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	0	60
Assets, Contract	0	1
Liabilities, Fair Value	234	0
Liabilities, Contract	1	0
Equity Futures Contracts [Member] \| Receivable from Brokers, Dealers and Clearing Organizations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	2,250	1,790
Assets, Contract	2,208	2,590
Liabilities, Fair Value	894	2,047
Liabilities, Contract	3,429	3,085
Equity Swap Contracts [Member] \| Receivable from Brokers, Dealers and Clearing Organizations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	46	98
Assets, Contract	1	1
Liabilities, Fair Value	0	13
Liabilities, Contract	0	1
Listed Options [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	87,412	144,586
Liabilities, Fair Value	78,427	115,362
Listed Options [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	87,412	144,586
Assets, Contract	261,752	426,747
Liabilities, Fair Value	78,427	115,362
Liabilities, Contract	256,523	437,383
Fixed Income Futures Contracts [Member] \| Receivable from Brokers, Dealers and Clearing Organizations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	4,409	6,432
Assets, Contract	6,896	11,901
Liabilities, Fair Value	8,061	6,891
Liabilities, Contract	11,399	10,628
Commodity Futures Contracts [Member] \| Receivable from Brokers, Dealers and Clearing Organizations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value	21,535	15,246
Assets, Contract	12,459	8,894
Liabilities, Fair Value	$22,332	$14,847
Liabilities, Contract	12,050	9,105

Derivative_Financial_Instrumen3

Derivative Financial Instruments - Gain (Loss) Recognized (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	$13,849	($38,184)
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Foreign Currency Futures Contracts [Member] \| Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	1,008	2,842
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Foreign Currency Forward Contracts [Member] \| Investment Income and Other, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	-294	432
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Equity Futures Contracts [Member] \| Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	2,377	8,737
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Equity Swap Contracts [Member] \| Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	329	1,282
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Listed Options [Member] \| Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	-13,550	-73,876
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Fixed Income Futures Contracts [Member] \| Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	8,544	8,829
Derivative Instruments Not Designated as Hedging Instruments [Member] \| Commodity Futures Contracts [Member] \| Trading Revenues, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, Gain (Loss)	15,435	13,570
Derivative Instruments Designated as Hedging Instruments [Member] \| Foreign Currency Forward Contracts [Member] \| Accumulated Other Comprehensive Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments designated as hedging instruments	$0	$168

Derivative_Financial_Instrumen4

Derivative Financial Instruments - Netting Arrangements (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Gross Amounts of Recognized, Assets	$116,374	$169,424
Gross Amounts Offset in the Statements of Financial Condition, Assets	28,916	24,449
Net Amounts Presented in the Statements of Financial Condition, Assets	87,458	144,975
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount, Assets	87,458	144,975
Gross Amounts of Recognized Derivatives, Liabilities	111,227	139,811
Gross Amounts of Recognized Derivatives, Liabilities	32,566	24,449
Net Amounts Presented in the Statements of Financial Condition, Liabilities	78,661	115,362
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	14,841	17,359
Net Amount, Liabilities	63,820	98,003
Listed Options [Member]
Derivative [Line Items]
Gross Amounts of Recognized, Assets	87,412	144,586
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts Presented in the Statements of Financial Condition, Assets	87,412	144,586
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount, Assets	87,412	144,586
Gross Amounts of Recognized Derivatives, Liabilities	78,427	115,362
Gross Amounts of Recognized Derivatives, Liabilities	0	0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	78,427	115,362
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	14,841	17,359
Net Amount, Liabilities	63,586	98,003
Swaps [Member]
Derivative [Line Items]
Gross Amounts of Recognized, Assets	46	98
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	13
Net Amounts Presented in the Statements of Financial Condition, Assets	46	85
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount, Assets	46	85
Gross Amounts of Recognized Derivatives, Liabilities		13
Gross Amounts of Recognized Derivatives, Liabilities		13
Net Amounts Presented in the Statements of Financial Condition, Liabilities		0
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities		0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities		0
Net Amount, Liabilities		0
Futures [Member]
Derivative [Line Items]
Gross Amounts of Recognized, Assets	28,916	24,680
Gross Amounts Offset in the Statements of Financial Condition, Assets	28,916	24,436
Net Amounts Presented in the Statements of Financial Condition, Assets	0	244
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount, Assets	0	244
Gross Amounts of Recognized Derivatives, Liabilities	32,566	24,436
Gross Amounts of Recognized Derivatives, Liabilities	32,566	24,436
Net Amounts Presented in the Statements of Financial Condition, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	0
Net Amount, Liabilities	0	0
Foreign Currency Forward Contracts [Member]
Derivative [Line Items]
Gross Amounts of Recognized, Assets		60
Gross Amounts Offset in the Statements of Financial Condition, Assets		0
Net Amounts Presented in the Statements of Financial Condition, Assets		60
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets		0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets		0
Net Amount, Assets		60
Gross Amounts of Recognized Derivatives, Liabilities	234
Gross Amounts of Recognized Derivatives, Liabilities	0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	234
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0
Net Amount, Liabilities	$234

Collateralized_Transactions_Fi

Collateralized Transactions - Financial Instruments at Fair Value Received as Collateral that were Permitted to be Delivered or Repledged (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Collateralized Agreements [Abstract]
Collateral permitted to be delivered or repledged	$1,614,177	$1,586,700
Collateral that was delivered or repledged	1,508,355	1,485,267
Collateral permitted to be further repledged by the receiving counterparty	$239,757	$147,696

Collateralized_Transactions_Fi1

Collateralized Transactions - Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Repurchase Agreement Counterparty [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge	$1,614,177	$1,586,700
Right to deliver or repledge [Member]
Repurchase Agreement Counterparty [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge	506,532	536,124
Not having the right to deliver or repledge [Member]
Repurchase Agreement Counterparty [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	$951,587	$979,652

Collateralized_Transactions_Gr

Collateralized Transactions - Gross Amounts Offset (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	$116,374	$169,424
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount, Assets	87,458	144,975
Gross Amounts of Recognized Derivatives, Liabilities	111,227	139,811
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	0
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	14,841	17,359
Net Amount, Liabilities	63,820	98,003
Securities Borrowed [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	1,685,850	1,632,062
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts Presented in the Statements of Financial Condition, Assets	1,685,850	1,632,062
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	1,612,086	1,570,194
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	13,355	15,782
Net Amount, Assets	60,409	46,086
Receivable from Brokers, Dealers and Clearing Organizations [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	133,935	21,545
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts Presented in the Statements of Financial Condition, Assets	133,935	21,545
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	133,829	21,425
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	0
Net Amount, Assets	106	120
Securities Loaned [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liabilities	792,887	707,744
Gross Amounts of Recognized Derivatives, Liabilities	0	0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	792,887	707,744
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	760,775	682,389
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	13,355	15,782
Net Amount, Liabilities	18,757	9,573
Financial Instruments Sold Under Agreements to Repurchase [Member}
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liabilities	905,567	933,576
Gross Amounts of Recognized Derivatives, Liabilities	0	0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	905,567	933,576
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	905,508	933,560
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	0
Net Amount, Liabilities	59	16
Financial Instruments Owned and Pledged as Collateral [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	1,819,785	1,653,607
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts Presented in the Statements of Financial Condition, Assets	1,819,785	1,653,607
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	1,745,915	1,591,619
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	13,355	15,782
Net Amount, Assets	60,515	46,206
Gross Amounts of Recognized Derivatives, Liabilities	1,698,454	1,641,320
Gross Amounts of Recognized Derivatives, Liabilities	0	0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	1,698,454	1,641,320
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	1,666,283	1,615,949
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	13,355	15,782
Net Amount, Liabilities	$18,816	$9,589

Collateralized_Transactions_Ad

Collateralized Transactions - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015
Collateralized Agreements [Abstract]
Repurchase agreements and other secured financings, maturity (years)	1 year

Collateralized_Transactions_Ma

Collateralized Transactions - Maturities Schedule (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	$1,698,454	$1,641,320
Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	878,454	781,320
0 - 30 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	445,000	410,000
31 - 60 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	225,000	325,000
61 - 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	150,000	125,000
Securities Loaned [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	792,887	707,744
Securities Loaned [Member] \| Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	792,887	707,744
Securities Loaned [Member] \| 0 - 30 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Securities Loaned [Member] \| 31 - 60 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Securities Loaned [Member] \| 61 - 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	0	0
Financial Instruments Sold Under Agreements to Repurchase [Member}
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	905,567	933,576
Financial Instruments Sold Under Agreements to Repurchase [Member} \| Overnight [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	85,567	73,576
Financial Instruments Sold Under Agreements to Repurchase [Member} \| 0 - 30 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	445,000	410,000
Financial Instruments Sold Under Agreements to Repurchase [Member} \| 31 - 60 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	225,000	325,000
Financial Instruments Sold Under Agreements to Repurchase [Member} \| 61 - 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Assets sold under agreements to repurchase	$150,000	$125,000

Receivable_from_and_Payable_to2

Receivable from and Payable to Brokers, Dealers, and Clearing Organizations - Schedule of Amounts Receivable from and Payable to Brokers, Dealers and Clearing Organizations (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Receivable/Payable:
Clearing organizations and other	$622,225	$1,073,480
Financial Instruments Purchased Under Agreement to Resell	133,935	21,545
Securities failed to deliver	162,346	93,808
Total Receivable	918,506	1,188,833
Clearing organizations and other	485,866	350,627
Securities failed to receive	52,977	325,462
Total Payable	$538,843	$676,089

Investments_Details

Investments (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments [Abstract]
Investments accounted for under the equity method	$90,369	$86,328
Investments held at fair value	4,738	4,435
Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	8,948	8,949
Total Strategic investments	104,055	99,712
Other investments	1,569	1,014
Total Investments	$105,624	$100,726

Investments_Additional_Informa

Investments - Additional Information (Details) (USD $)	3 Months Ended
Investments - Additional Information (Details) (USD $)	Mar. 31, 2015	Mar. 31, 2014
Schedule of Equity Method Investments [Line Items]
Proceeds and distributions from investments	$2,013,000	$42,365,000
Investment income and other, net	387,423,000	12,155,000
Investment Income and Other, Net [Member]
Schedule of Equity Method Investments [Line Items]
Income from equity method investments	2,200,000	13,000,000
BATS [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of equity in common stock of private companies cover under strategic investment	16.70%
Percentage of voting interest held	19.90%
BATS and DE [Member]
Schedule of Equity Method Investments [Line Items]
Proceeds and distributions from investments	42,200,000
Merger related transaction costs		6,600,000
BATS and DE [Member] \| Investment Income and Other, Net [Member]
Schedule of Equity Method Investments [Line Items]
Investment income and other, net		9,600,000
DE [Member]
Schedule of Equity Method Investments [Line Items]
Investment income and other, net		$16,200,000

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Goodwill and intangible assets impairment		$0
Weighted average useful life	5 years		5 years
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period	1 year
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period	8 years

Goodwill_and_Intangible_Assets3

Goodwill and Intangible Assets - Summary of Goodwill (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Goodwill	$17,311	$17,311
Market Making [Member]
Goodwill [Line Items]
Goodwill	16,404	16,404
Global Execution Services [Member]
Goodwill [Line Items]
Goodwill	$907	$907

Goodwill_and_Intangible_Assets4

Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$194,197,000	$192,272,000
Accumulated amortization	-64,969,000	-56,989,000
Net carrying amount	129,228,000	135,283,000
KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		34,696,000
Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	117,729,000	115,804,000
Accumulated amortization	-42,989,000	-36,562,000
Net carrying amount	74,740,000	79,242,000
Amortization period	3 years	3 years
Technology [Member] \| KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		13,100,000
Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	62,468,000	62,468,000
Accumulated amortization	-19,277,000	-18,110,000
Net carrying amount	43,191,000	44,358,000
Amortization period	7 years	7 years
Indefinite-lived intangible assets	6,800,000	6,800,000
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	13,000,000	13,000,000
Accumulated amortization	-2,528,000	-2,167,000
Net carrying amount	10,472,000	10,833,000
Amortization period	7 years	8 years
Customer Relationships [Member] \| KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		19,000,000
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,000,000	1,000,000
Accumulated amortization	-175,000	-150,000
Net carrying amount	825,000	850,000
Amortization period	8 years	9 years
Trade Names [Member] \| KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		2,600,000
Market Making [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	91,281,000	94,900,000
Market Making [Member] \| Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	48,090,000	50,542,000
Market Making [Member] \| Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	43,191,000	44,358,000
Electronic Execution Services [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	28,197,000	29,883,000
Electronic Execution Services [Member] \| Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	16,900,000	18,200,000
Electronic Execution Services [Member] \| Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	10,472,000	10,833,000
Electronic Execution Services [Member] \| Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	825,000	850,000
Corporate and Other [Member] \| Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$9,750,000	$10,500,000

Goodwill_and_Intangible_Assets5

Goodwill and Intangible Assets - Summary of Amortization Expense Relating to Intangible Assets (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense	$9,249	$8,494

Goodwill_and_Intangible_Assets6

Goodwill and Intangible Assets - Summary of Estimated Amortization Expense for Future Years (Detail) (USD $)	Mar. 31, 2015
In Thousands, unless otherwise specified	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
For the nine months ended December 31, 2015	$23,671
For the year ended December 31, 2016	30,679
For the year ended December 31, 2017	27,892
For the year ended December 31, 2018	15,788
For the year ended December 31, 2019	$6,211

Debt_Summary_of_LongTerm_Debt_

Debt - Summary of Long-Term Debt (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Jun. 05, 2013	Mar. 10, 2015
Debt Disclosure [Abstract]
Cash Convertible Senior Subordinated Notes, Carrying Amount	$0	$117,259,000
8.25% Senior Secured Notes, Carrying Amount	305,000,000	305,000,000
6.875% Senior Secured Notes, Carrying Amount	494,847,000	0
Total Debt, Carrying Amount	799,847,000	422,259,000
Cash Convertible Senior Subordinated Notes, Fair Value	0	116,819,000
8.25% Senior Secured Notes, Fair Value	329,747,000	309,194,000
6.875% Senior Secured Notes, Fair Value	484,950,000	0
Total Debt, Fair Value	814,697,000	426,013,000
Secured Debt [Member] \| 8.25% Senior Secured Notes [Member]
Debt Disclosure [Abstract]
8.25% Senior Secured Notes, Carrying Amount			$305,000,000
Debt Instrument [Line Items]
Interest rate	8.25%	8.25%	8.25%
Senior Secured Notes [Member] \| 6.875% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Interest rate	6.88%			6.88%

Debt_Debt_Redemption_Details

Debt - Debt Redemption (Details) (Senior Secured Notes [Member], 6.875% Senior Secured Notes [Member])	0 Months Ended	3 Months Ended
	Mar. 10, 2015	Mar. 31, 2015
Debt Instrument [Line Items]
Percentage	106.88%
2017 [Member]
Debt Instrument [Line Items]
Percentage		103.44%
2018 [Member]
Debt Instrument [Line Items]
Percentage		101.72%
2019 and thereafter [Member]
Debt Instrument [Line Items]
Percentage		100.00%

Debt_Recorded_Expenses_with_Re

Debt - Recorded Expenses with Respect to Long-term Debt (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Line of Credit Facility [Line Items]
Interest expense	$8,158	$9,416
Amortization of debt issuance cost	934	8,766
Commitment fee	394	397
Total	9,486	18,579
Writedown of debt issuance costs [Member]
Line of Credit Facility [Line Items]
Amortization of debt issuance cost		7,600
Other Expense [Member]
Line of Credit Facility [Line Items]
Amortization of debt issuance cost		$1,600

Debt_Additional_Information_De

Debt - Additional Information (Details) (USD $)

0 Months Ended

1 Months Ended

3 Months Ended

12 Months Ended

0 Months Ended

4 Months Ended

0 Months Ended

Mar. 16, 2015

Mar. 31, 2010

Mar. 31, 2015

Mar. 31, 2014

Jun. 30, 2014

Dec. 31, 2013

Jul. 01, 2013

Mar. 13, 2015

Apr. 13, 2015

Jun. 05, 2013

Mar. 10, 2015

Dec. 31, 2014

Line of Credit Facility [Line Items]

Secured debt

$305,000,000

Issue of cash convertible senior subordinated notes

375,000,000

Debt maturity date

15-Mar-15

Payment of convertible notes

119,300,000

Payment of convertible notes, principal amount

117,300,000

Payment of convertible notes, interest portion

2,100,000

Notes bear interest rate per year

3.50%

Date of commencing of notes

15-Sep-10

Funds held in escrow

330,163,000

Proceeds from issuance of debt, net

494,810,000

Writedown of capitalized debt costs

7,557,000

First Lien Credit Facility [Member]

Line of Credit Facility [Line Items]

Cash held in collateral account

117,300,000

First Lien Credit Facility [Member] | Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

Line of Credit Facility [Line Items]

Term credit agreement

535,000,000

Principal payments

185,000,000

50,000,000

300,000,000

Writedown of capitalized debt costs

7,600,000

Revolving Credit Facility [Member] | Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

Line of Credit Facility [Line Items]

Revolving credit facility amount

50,000,000

OCTEG-KCA [Member] | Revolving Credit Facility [Member]

Line of Credit Facility [Line Items]

Term credit agreement

450,000,000

Revolving credit facility amount

750,000,000

Number of debt instruments

Number of borrowing base

Increase in line of credit facility, maximum borrowing capacity

300,000,000

Commitment fee percentage of average daily amount unused portion of revolving credit agreement

0.35%

Debt issuance costs incurred

1,200,000

OCTEG-KCA [Member] | Revolving Credit Facility [Member] | Borrowing Base A [Member]

Line of Credit Facility [Line Items]

Percentage points added to interest rate base

1.75%

OCTEG-KCA [Member] | Revolving Credit Facility [Member] | Borrowing Base B Loan [Member]

Line of Credit Facility [Line Items]

Percentage points added to interest rate base

2.25%

OCTEG-KCA [Member] | Swingline Facilty [Member]

Line of Credit Facility [Line Items]

Revolving credit facility amount

50,000,000

Secured Debt [Member] | 8.25% Senior Secured Notes [Member]

Line of Credit Facility [Line Items]

Secured debt

305,000,000

Interest rate

8.25%

Escrow agent fees and expenses

3,000,000

Redemption notice, period

30 days

Aggregate principal amount

305,000,000

Secured Debt [Member] | 8.25% Senior Secured Notes [Member] | Subsequent Event [Member]

Line of Credit Facility [Line Items]

Payment of secured debt

329,700,000

Interest rate

8.25%

Funds held in escrow

330,200,000

Accrued interest for the period

8,200,000

Make-whole premium percentage

4.13%

Write off of charges for make-whole premium

16,500,000

Other miscellaneous charges

400,000

Writedown of capitalized debt costs

8,500,000

Secured Debt [Member] | 8.25% Senior Secured Notes [Member] | Prior to June 15, 2015 [Member]

Line of Credit Facility [Line Items]

Redemption price percentage

100.00%

Senior Secured Notes [Member] | 6.875% Senior Secured Notes [Member]

Line of Credit Facility [Line Items]

Interest rate

6.88%

Redemption price percentage

106.88%

Issuance of debt, percentage

98.96%

Proceeds from issuance of debt, net

494,800,000

Yield to maturity, percentage

7.58%

7.08%

Percentage in voting equity interests of controlled foreign subsidiaries

66.00%

Redemption period

12 months

Maximum redemption percentage of principal amount

40.00%

Senior Secured Notes [Member] | 6.875% Senior Secured Notes [Member] | Other Assets [Member]

Line of Credit Facility [Line Items]

Deferred debt costs

12,400,000

Senior Secured Notes [Member] | 6.875% Senior Secured Notes [Member] | Minimum [Member]

Line of Credit Facility [Line Items]

Redemption notice, period

30 days

Senior Secured Notes [Member] | 6.875% Senior Secured Notes [Member] | Maximum [Member]

Line of Credit Facility [Line Items]

Redemption notice, period

60 days

Senior Secured Notes [Member] | 6.875% Senior Secured Notes [Member] | Jefferies LLC [Member]

Line of Credit Facility [Line Items]

Aggregate principal amount

$500,000,000

Senior Secured Notes [Member] | 6.875% Senior Secured Notes [Member] | Prior to March 15, 2017 [Member]

Line of Credit Facility [Line Items]

Redemption price percentage

100.00%

Senior Secured Notes [Member] | 6.875% Indenture [Member]

Line of Credit Facility [Line Items]

Interest rate

6.88%

Related_Parties_Additional_Inf

Related Parties - Additional Information (Details) (USD $)	3 Months Ended		1 Months Ended
Related Parties - Additional Information (Details) (USD $)	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015
Related Party Transaction [Line Items]
Professional fees	$11,181,000	$5,402,000
KCG Hotspot [Member]
Related Party Transaction [Line Items]
Gain from sale of business	373,800,000
Professional fees	6,700,000
Investment Income and Other, Net [Member] \| KCG Hotspot [Member]
Related Party Transaction [Line Items]
Gain from sale of business			385,000,000
Related Parties [Member]
Related Party Transaction [Line Items]
Number of equity method investees	4		4
Financing and advisory fees	16,800,000
Professional fees	5,507,000	0
Related Parties [Member] \| Other Assets [Member]
Related Party Transaction [Line Items]
Deferred debt costs	$11,300,000		$11,300,000
Related Parties [Member] \| Minimum [Member] \| Common Class A [Member]
Related Party Transaction [Line Items]
Number of beneficial owners	2		2
Noncontrolling interest, ownership percentage by noncontrolling owners	10.00%		10.00%

Related_Parties_Summary_of_Bal

Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Revenues
Commissions and fees	$99,961	$112,257
Trading revenues, net	208,795	258,297
Expenses
Execution and clearance fees	68,473	75,501
Communications and data processing	33,764	36,796
Payment for order flow	15,221	22,032
Collateralized financing interest	8,456	6,162
Professional fees	11,181	5,402
Other expense	7,808	8,643
Assets
Securities borrowed	1,685,850		1,632,062
Receivable from brokers, dealers and clearing organizations	918,506		1,188,833
Other assets	217,407		137,645
Liabilities
Securities loaned	792,887		707,744
Payable to brokers, dealers and clearing organizations	538,843		676,089
Related Parties [Member]
Revenues
Commissions and fees	3,492	5,146
Trading revenues, net	2,101	814
Interest, net	226	188
Total revenues from related parties	5,819	6,148
Expenses
Execution and clearance fees	-4,628	-4,011
Communications and data processing	1,110	0
Payment for order flow	1,180	0
Collateralized financing interest	113	159
Professional fees	5,507	0
Other expense	621	400
Total expenses incurred with respect to related parties	3,903	-3,452
Assets
Securities borrowed	41,600		26,110
Receivable from brokers, dealers and clearing organizations	92,739		20,075
Other assets	12,247	0
Liabilities
Securities loaned	3,543		7,376
Payable to brokers, dealers and clearing organizations	64,757		8,509
Accrued expenses and other liabilities	$4,693		$5,667

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)	0	0
Number of shares exercised	0	0
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Unamortized compensation related to restricted awards outstanding	59.2
Cost of unvested awards expected to be recognized over a weighted average life, years	1 year 7 months 20 days
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Stock Options [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	5 years
Stock Options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period	10 years
SARs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SARs granted (in shares)	0	0
Number of SARs exercised during period (in shares)	0	0
Unamortized compensation costs	2.3
Award expiration period	7 months 24 days
Incentive Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred compensation payable	3.1		$2.90
KCG Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	28,900,000
Number of shares available for grant	15,100,000

StockBased_Compensation_Compen

Stock-Based Compensation - Compensation Expense Relating to Restricted Awards (Detail) (Restricted Stock [Member], USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock award compensation expense	$14,255	$16,238
Income tax benefit	$5,417	$6,170

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Restricted Awards Activity (Detail) (Restricted Stock Units [Member], USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015
Restricted Stock Units [Member]
Number of Units
Beginning Balance (in shares)	9,147
Granted (in shares)	2,385
Vested (in shares)	-2,055
Forfeited (in shares)	-322
Ending Balance (in shares)	9,155
Weighted- Average Grant date Fair Value
Beginning Balance (in dollars per share)	$10.77
Granted (in dollars per share)	$12.22
Vested (in dollars per share)	$11.04
Forfeited (in dollars per share)	$11.25
Ending Balance (in dollars per share)	$11.07

StockBased_Compensation_Compen1

Stock-Based Compensation - Compensation Expense Relating to Stock Options (Detail) (Stock Options and SARs [Member], USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Stock Options and SARs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option and SAR compensation expense	$1,013	$961
Income tax benefit	$385	$365

StockBased_Compensation_Summar1

Stock-Based Compensation - Summary of Stock Option and SAR Activity (Detail) (USD $)	3 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Number of Stock Awards
Exercised (in shares)	0	0
Stock Options and SARs [Member]
Number of Stock Awards
Beginning Balance (in shares)	4,691,000
Granted at market value (in shares)	0
Exercised (in shares)	0
Forfeited or expired (in shares)	-84,000
Ending Balance (in shares)	4,607,000
Exercisable (in shares)	1,782,000
Available for future grants (in shares)	15,086,000
Weighted- Average Exercise Price
Beginning Balance (in dollars per share)	17.41
Granted at market value (in dollars per share)	0
Exercised (in dollars per share)	0
Forfeited or expired (in dollars per share)	17.1
Ending Balance (in dollars per share)	17.42
Exercisable (in dollars per share)	20.72
Aggregate Intrinsic Value of Awards, Outstanding	4,687
Aggregate Intrinsic Value of Awards, Exercisable	1,640
Weighted-Average Remaining Life, Outstanding	3 years 1 month 24 days
Weighted-Average Remaining Life, Exercisable	2 years 11 months 12 days
SARs [Member]
Number of Stock Awards
Beginning Balance (in shares)			1,700,000
Ending Balance (in shares)	1,700,000		1,700,000

StockBased_Compensation_Incent

Stock-Based Compensation - Incentive Units (Details) (Incentive Units [Member], Common Class A [Member])	3 Months Ended
	Mar. 31, 2015
Incentive Units [Member] \| Common Class A [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]
Beginning balance (in shares)	38,000
Issued (in shares)	0
Vested (in shares)	0
Exercised (in shares)	0
Canceled (in shares)	0
Ending balance (in shares)	38,000

StockBased_Compensation_Compen2

Stock-Based Compensation - Compensation Expense (Details) (Incentive Units [Member], USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Incentive Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$153	($174)

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
Income Taxes - Additional Information (Detail) (USD $)	Dec. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Income Tax Contingency [Line Items]
NOLs, valuation allowance		$0
General business carry forward		4,400,000
Unrecognized tax benefits that would impact effective tax rate if recognized		2,300,000	1,500,000
U.S. Federal [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		133,500,000
Deferred tax asset not subject to expiration		46,700,000
Deferred tax asset, valuation allowance, not subject to expiration		6,800,000
Deferred income tax asset		19,500,000
U.S. Federal [Member] \| Knight [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		28,500,000
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		1,280,000,000
Deferred tax asset not subject to expiration		18,500,000
Deferred tax asset, valuation allowance, not subject to expiration		8,100,000
Valuation allowance reversed	10,800,000
AMT Carryforward		6,800,000
Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		86,400,000
Deferred income tax asset		19,500,000
Foreign Tax Authority [Member] \| Knight [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		$65,700,000

Income_Taxes_Effective_Income_

Income Taxes - Effective Income Tax Rate Reconciliation (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	35.00%	35.00%
Federal income tax effect	3.60%	2.20%
Nondeductible expenses	0.00%	0.30%
Foreign Taxes	0.00%	0.10%
Other, net	0.00%	0.20%
Actual income tax rate	38.60%	37.80%

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$2,133	$1,401
Other comprehensive income (loss)	-435	64
Ending Balance	1,698	1,465
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	352	36
Other comprehensive income (loss)	188	-137
Ending Balance	540	-101
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	1,781	1,365
Other comprehensive income (loss)	-623	201
Ending Balance	$1,158	$1,566

Writedowns_and_Other_Charges_A

Writedowns and Other Charges - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Writedown Of Asset And Lease Loss Accrual [Line Items]
Writedown of capitalized debt costs	$0	$7,557,000
Secured Debt [Member]
Writedown Of Asset And Lease Loss Accrual [Line Items]
Early principal payments		185,000,000
Writedown of capitalized debt costs		$7,600,000

Earnings_Per_Share_Additional_

Earnings Per Share - Additional Information (Detail)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Options excluded (in shares)	18.3	28.4

Earnings_Per_Share_Reconciliat

Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted (Loss) Earnings Per Computations (Detail) (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Numerator / net income
Income from continuing operations and shares used in basic calculations	$249,301	$36,917
Income from continuing operations and shares used in diluted calculations	$249,301	$36,917
Denominator / shares
Income from continuing operations and shares used in basic calculations (in shares)	110,782	115,569
Income from continuing operations and shares used in diluted calculations (in shares)	113,615	117,898
Basic earnings per share from continuing operations (in dollars per share)	$2.25	$0.32
Diluted earnings per common share from continuing operations (in dollars per share)	$2.19	$0.31
Restricted Awards [Member]
Denominator / shares
Effect of dilutive stock based awards (in shares)	2,410	2,233
Stock Options and SARs [Member]
Denominator / shares
Effect of dilutive stock based awards (in shares)	286	96
Warrants [Member]
Denominator / shares
Effect of dilutive stock based awards (in shares)	137	0

Significant_Clients_Additional

Significant Clients - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
	Client	Client
Risks and Uncertainties [Abstract]
Number of significant clients	0	0
Percentage of dollar value traded	10.00%	10.00%

Commitments_and_Contingent_Lia2

Commitments and Contingent Liabilities - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Aug. 01, 2012	Mar. 02, 2015	Dec. 31, 2014
	Mar. 31, 2015	Mar. 31, 2014	lawsuit	Mar. 02, 2015	Dec. 31, 2014
Loss Contingencies [Line Items]
Rental expense	$4,600,000	$5,100,000
Letters of credit held in escrow	330,163,000				0
Compensation guarantees payable	0	0
Guarantee Obligations [Member]
Loss Contingencies [Line Items]
Letters of credit obligation	11,600,000
Revolving Credit Facility [Member]
Loss Contingencies [Line Items]
Weighted average interest rate	3.46%
Debt term	3 years
Technology Issue [Member]
Loss Contingencies [Line Items]
Number of putative class action lawsuits			2
Number of class action lawsuit voluntarily dismissed			1
Number of derivative lawsuits			1
Raise In equity financing			400,000,000
Fernandez Action [Member]
Loss Contingencies [Line Items]
Litigation settlement, amount				$13,000,000

Commitments_and_Contingent_Lia3

Commitments and Contingent Liabilities - Schedule of Capital Lease and Contract Obligations (Details) (USD $)	Mar. 31, 2015
In Thousands, unless otherwise specified	Mar. 31, 2015
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Nine months ended December 31, 2015	$2,535
2016	2,126
2017	620
Total	$5,281

Commitments_and_Contingent_Lia4

Commitments and Contingent Liabilities - Schedule of Interest Expense (Details) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Interest expense - Capital leases	$68	$88

Commitments_and_Contingent_Lia5

Commitments and Contingent Liabilities - Schedule of Operating Lease and Contract Obligations (Detail) (USD $)	Mar. 31, 2015
In Thousands, unless otherwise specified	Mar. 31, 2015
Gross Lease Obligations [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Nine months ending December 31, 2015	$21,159
Year ending December 31, 2016	27,934
Year ending December 31, 2017	26,395
Year ending December 31, 2018	25,550
Year ending December 31, 2019	23,671
Thereafter through December 31, 2027	64,589
Total	189,298
Sublease Income [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Nine months ending December 31, 2015	3,903
Year ending December 31, 2016	4,994
Year ending December 31, 2017	4,508
Year ending December 31, 2018	4,211
Year ending December 31, 2019	3,630
Thereafter through December 31, 2027	10,244
Total	31,490
Net Lease Obligations [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Nine months ending December 31, 2015	17,256
Year ending December 31, 2016	22,940
Year ending December 31, 2017	21,887
Year ending December 31, 2018	21,339
Year ending December 31, 2019	20,041
Thereafter through December 31, 2027	54,345
Total	$157,808

Financial_instruments_with_Off1

Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Loans outstanding	$799,847,000	$422,259,000
Executive Director or Officer [Member]
Derivatives, Fair Value [Line Items]
Loans outstanding	$0

Business_Segments_Detail

Business Segments (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$696,156	$383,657
Pre-tax earnings	406,128	59,384
Total assets	7,332,051	7,030,276	6,830,654
Market Making [Member]
Segment Reporting Information [Line Items]
Revenues	224,548	277,346
Pre-tax earnings	39,340	76,032
Total assets	4,700,273	3,931,292
Global Execution Services [Member]
Segment Reporting Information [Line Items]
Revenues	464,266	87,220
Pre-tax earnings	381,058	2,016
Total assets	783,501	1,264,245
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	7,342	19,091
Pre-tax earnings	-14,270	-18,664
Total assets	$1,848,277	$1,834,739

Business_Segments_Additional_I

Business Segments - Additional Information (Details) (KCG Hotspot [Member], USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Segment Reporting Information [Line Items]
Gain from sale of business	$373.80
Global Execution Services [Member] \| Revenues [Member]
Segment Reporting Information [Line Items]
Gain from sale of business	385
Global Execution Services [Member] \| Pre-tax Earnings [Member]
Segment Reporting Information [Line Items]
Gain from sale of business	$373.80

Subsequent_Events_Details

Subsequent Events (Details) (USD $)	3 Months Ended		0 Months Ended
Share data in Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	4-May-15	Apr. 13, 2015	Dec. 31, 2014	Jun. 05, 2013	Apr. 02, 2015	Apr. 30, 2015
Subsequent Event [Line Items]
Funds held in escrow	$330,163,000				$0
Debt	799,847,000				422,259,000
Writedown of capitalized debt costs	0	7,557,000
Secured Debt [Member] \| 8.25% Senior Secured Notes [Member]
Subsequent Event [Line Items]
Interest rate	8.25%				8.25%	8.25%
Subsequent Event [Member]
Subsequent Event [Line Items]
Authorized amount to be repurchased							400,000,000
Minimum tender offer period			20 days
Debt				495,000,000
Subsequent Event [Member] \| Secured Debt [Member] \| 8.25% Senior Secured Notes [Member]
Subsequent Event [Line Items]
Funds held in escrow				330,200,000
Interest rate				8.25%
Write off of charges for make-whole premium				16,500,000
Writedown of capitalized debt costs				8,500,000
Subsequent Event [Member] \| Common Class A [Member]
Subsequent Event [Line Items]
Remaining authorized repurchase amount							55,000,000
Subsequent Event [Member] \| Common Class A [Member] \| Modified Dutch Auction Tender Offer [Member]
Subsequent Event [Line Items]
Authorized amount to be repurchased			330,000,000
Remaining authorized repurchase amount								$70,000,000
Number of shares to be repurchased, percentage of total shares outstanding								22.00%
Subsequent Event [Member] \| Common Class A [Member] \| Modified Dutch Auction Tender Offer [Member] \| Minimum [Member]
Subsequent Event [Line Items]
Share price (in dollars per share)			13.5
Number of shares to be repurchased								23.6
Subsequent Event [Member] \| Common Class A [Member] \| Modified Dutch Auction Tender Offer [Member] \| Maximum [Member]
Subsequent Event [Line Items]
Share price (in dollars per share)			14