KCG Inactive 10-K2 Mar 152014 FY Annual reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Jun. 30, 2014	Feb. 25, 2015
Entity Information [Line Items]
Entity Registrant Name	KCG HOLDINGS, INC.
Entity Central Index Key	1569391
Document Type	10-K
Document Period End Date	31-Dec-14
Amendment Flag	FALSE
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float		$866.80
Common Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			118,439,414
Common Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding			0

CONSOLIDATED_STATEMENTS_OF_FIN

CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets
Cash and cash equivalents	$578,768	$674,281
Cash and cash equivalents segregated under federal and other regulations	3,361	183,082
Financial instruments owned, at fair value, including securities pledged to counterparties that had the right to deliver or repledge of $536,124 at December 31, 2014 and $552,242 at December 31, 2013:
Equities	2,479,910	2,298,785
Listed options	144,586	339,798
Debt securities	82,815	83,256
Other financial instruments	60	0
Total financial instruments owned, at fair value	2,707,371	2,721,839
Collateralized agreements:
Securities borrowed	1,632,062	1,357,387
Receivable from brokers, dealers and clearing organizations	1,188,833	1,257,251
Fixed assets and leasehold improvements, less accumulated depreciation and amortization	134,051	146,668
Investments	100,726	125,413
Goodwill and Intangible assets, less accumulated amortization	152,594	208,806
Deferred tax asset, net	154,759	175,639
Assets of business held for sale	40,484	0
Other assets	137,645	146,638
Total assets	6,830,654	6,997,004
Financial instruments sold, not yet purchased, at fair value:
Equities	2,069,342	1,851,006
Listed options	115,362	252,282
Debt securities	101,003	57,198
Other financial instruments	0	5,014
Total financial instruments sold, not yet purchased, at fair value	2,285,707	2,165,500
Collateralized financings:
Securities loaned	707,744	733,230
Financial instruments sold under agreements to repurchase	933,576	640,950
Total collateralized financings	1,641,320	1,374,180
Payable to brokers, dealers and clearing organizations	676,089	474,108
Payable to customers	22,110	481,041
Accrued compensation expense	114,559	149,430
Accrued expenses and other liabilities	136,977	175,910
Capital lease obligations	6,700	10,039
Liabilities of business held for sale	2,356	0
Debt	422,259	657,259
Total liabilities	5,308,077	5,487,467
Commitments and Contingent Liabilities (Note 22)
Equity
Shares authorized: 1,000,000 at December 31, 2014 and December 31, 2013; Shares issued: 127,508 at December 31, 2014 and 123,317 at December 31, 2013; Shares outstanding: 116,860 at December 31, 2014 and 122,238 at December 31, 2013	1,275	1,233
Additional paid-in capital	1,369,298	1,306,549
Retained earnings	272,780	211,678
Treasury stock, at cost; 10,649 shares at December 31, 2014 and 1,079 shares at December 31, 2013	-122,909	-11,324
Accumulated other comprehensive income	2,133	1,401
Total equity	1,522,577	1,509,537
Total liabilities and equity	$6,830,654	$6,997,004

CONSOLIDATED_STATEMENTS_OF_FIN1

CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Financial instruments owned, at fair value, securities pledged	$536,124	$552,242
Common stock, shares authorized	1,000,000,000	1,000,000,000
Common stock, shares issued	127,508,000	123,317,000
Common stock, shares outstanding	116,860,000	122,238,000
Treasury stock, shares	10,649,000	1,079,000

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Trading revenues, net	$837,357	$628,304	$421,063
Commissions and fees	437,022	275,474	105,518
Interest, net	621	-537	-2,357
Investment income and other, net	41,232	124,095	27,010
Total revenues	1,316,232	1,027,336	551,234
Expenses
Employee compensation and benefits	437,269	349,192	157,855
Execution and clearance fees	305,177	246,414	185,790
Communications and data processing	150,595	123,552	90,623
Depreciation and amortization	81,448	55,570	34,938
Payments for order flow	70,183	35,711	2,964
Occupancy and equipment rentals	32,707	24,812	12,804
Debt interest expense	32,456	34,938	2,665
Collateralized financing interest	27,860	9,847	0
Professional fees	25,596	46,662	14,072
Business development	9,763	4,609	23
Writedown of assets and lease loss accrual, net	8,625	14,748	0
Writedown of capitalized debt costs	9,552	13,209	0
Other	39,814	43,094	23,073
Total expenses	1,231,045	1,002,358	524,807
Income from continuing operations before income taxes	85,187	24,978	26,427
Income tax expense (benefit)	22,753	-101,114	10,276
Income from continuing operations, net of tax	62,434	126,092	16,151
(Loss) Income from discontinued operations, net of tax	-1,332	80	0
Net income	61,102	126,172	16,151
Loss from continuing operations allocated to preferred and participating units	0	-21,565	1,092
Net income attributable to common shareholders	$61,102	$147,737	$15,059
Basic earnings per share from continuing operations (in dollars per share)	$0.55	$1.84	$0.31
Diluted earnings per share from continuing operations (in dollars per share)	$0.54	$1.82	$0.31
Basic loss per share from discontinued operations (in dollars per share)	($0.01)	$0	$0
Diluted loss per share from discontinued operations (in dollars per share)	($0.01)	$0	$0
Basic earnings per share (in dollars per share)	$0.54	$1.84	$0.31
Diluted earnings per share (in dollars per share)	$0.52	$1.82	$0.31
Shares used in computation of basic earnings (loss) per share	112,854	80,143	48,970
Shares used in computation of diluted earnings (loss) per share	116,534	81,015	48,970

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$61,102	$126,172	$16,151
Other comprehensive income (loss):
Unrealized gain (loss) on available for sale securities, net of tax	316	-114,283	114,319
Cumulative translation adjustment, net of tax	416	1,365	0
Comprehensive income	$61,834	$13,254	$130,470

CONSOLIDATED_STATEMENTS_OF_CHA

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $)	Total	Redeemable Preferred Member's Equity [Member]	Members Equity [Member]	Unrecognized Compensation [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]
In Thousands, except Share data, unless otherwise specified	Total	Redeemable Preferred Member's Equity [Member]	Members Equity [Member]	Unrecognized Compensation [Member]	Common Class A [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income [Member]
Balance at Dec. 31, 2011								$0
Balance at Dec. 31, 2011	622,996	314,490	666,764	-43,768	0	0	0		0
Balance (in shares) at Dec. 31, 2011					0
Balance (in shares) at Dec. 31, 2011								0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributions	37,524		12,695	24,829
Repurchase of Membership interests	-111,525		-111,525
Distributions	-24,793	-3,351	-24,793
Net income	16,151		16,151						114,319
Comprehensive income	130,470
Balance at Dec. 31, 2012								0
Balance at Dec. 31, 2012	654,672	311,139	559,292	-18,939	0	0	0		114,319
Balance (in shares) at Dec. 31, 2012					0
Balance (in shares) at Dec. 31, 2012								0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Contributions	30,278		11,339	18,939
Repurchase of Membership interests	-5,833		-5,833
Distributions	-2,054		-2,054
Net income	-89,305		-89,305
Unrealized gain on available for sale securities, net	4,550								4,550
Cumulative translation adjustment, net	-403								-403
Modification of redemption value	21,565	-21,565	21,565
Balance at Jun. 30, 2013	613,470	289,574	495,004	0	0	0	0		118,466
Balance at Jun. 30, 2013								0
Balance (in shares) at Jun. 30, 2013					0
Balance (in shares) at Jun. 30, 2013								0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	209,393						209,393
Unrealized gain on available for sale securities, net	36								36
Cumulative translation adjustment, net	1,768								1,768
Gain on investment in Knight Capital Group, Inc.	9,103								9,103
Reclassification of investment in Knight out of other comprehensive income	-127,972								-127,972
Equity issued to General Atlantic	55,000		55,000
Exchange of membership interests for shares of KCG Class A Common Stock, shares					75,868,000
Exchange of membership interests for shares of KCG Class A Common Stock	205,172	-289,574	-550,004		759	754,417
Exchange of membership interests for warrants to purchase KCG Class A Common Stock	74,896					74,896
Issuance of KCG Class A Common Stock to Knight stockholder, shares					41,889,000
Issuance of KCG Class A Common Stock to Knight stockholder	453,838				419	453,419
Change in estimated distribution payable to members	1,757					1,757
KCG Class A Common Stock repurchased, shares								-1,079,000
KCG Class A Common Stock repurchased	-11,324							-11,324
Stock-based compensation, shares					5,560,000
Stock-based compensation	22,115				55	22,060
Balance at Dec. 31, 2013 (As Reported [Member])								-11,324
Balance at Dec. 31, 2013 (As Reported [Member])	1,507,252				1,233	1,306,549	209,393		1,401
Balance (Change to Equity Method of Accounting [Member])	2,285						2,285
Balance at Dec. 31, 2013	-11,324							-11,324
Balance at Dec. 31, 2013	1,509,537	0	0	0	1,233	1,306,549	211,678		1,401
Balance (in shares) at Dec. 31, 2013 (As Reported [Member])								-1,079,000
Balance (in shares) at Dec. 31, 2013 (As Reported [Member])					123,317,000
Balance (in shares) at Dec. 31, 2013					123,317,000
Balance (in shares) at Dec. 31, 2013	-1,079,000							-1,079,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	61,102					0	61,102
Comprehensive income	61,834
Cumulative translation adjustment, net	416								416
KCG Class A Common Stock repurchased, shares								-9,570,000
KCG Class A Common Stock repurchased	-111,585							-111,585
Stock-based compensation, shares					4,191,000
Stock-based compensation	61,907				42	61,865
Unrealized gain on available for sale securities, net	316								316
Income tax provision-stock based compensation	884					884
Balance at Dec. 31, 2014	-122,909							-122,909
Balance at Dec. 31, 2014	$1,522,577	$0	$0	$0	$1,275	$1,369,298	$272,780		$2,133
Balance (in shares) at Dec. 31, 2014					127,508,000
Balance (in shares) at Dec. 31, 2014	-10,649,000							-10,649,000

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net income	$61,102,000	$126,172,000	$16,151,000
(Loss) Income from discontinued operations, net of tax	-1,332,000	80,000	0
Income from continuing operations, net of tax	62,434,000	126,092,000	16,151,000
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by operating activities
Non-cash gain from sale of Futures Commission Merchant	-116,000	0	0
Non-cash gain on Knight Common Stock	0	-127,972,000	0
Deferred tax benefit	19,397,000	-103,499,000	0
Depreciation and amortization	81,448,000	55,570,000	34,938,000
Stock and unit-based compensation	58,940,000	64,286,000	12,320,000
Writedown and amortization of debt offering costs	13,217,000	17,332,000	276,000
Writedown of assets and lease loss accrual, net	8,625,000	14,748,000	0
Unrealized (gain) loss on investments	-36,456,000	-4,539,000	-25,754,000
Deferred rent	930,000	58,000	0
Operating activities from discontinued operations	-1,073,000	6,952,000	0
(Increase) decrease in operating assets
Cash and cash equivalents segregated under federal and other regulations	-54,955,000	19,963,000	0
Financial instruments owned, at fair value	14,468,000	-129,035,000	-414,537,000
Securities borrowed	-274,675,000	-146,145,000	-30,679,000
Receivable from brokers, dealers and clearing organizations	-321,087,000	252,266,000	36,439,000
Other assets	-6,459,000	25,785,000	1,332,000
(Decrease) increase in operating liabilities
Financial instruments sold, not yet purchased, at fair value	120,207,000	139,951,000	0
Securities loaned	-25,486,000	106,339,000	0
Financial instruments sold under agreements to repurchase	292,625,000	95,950,000	372,023,000
Payable to brokers, dealers and clearing organizations	268,563,000	-185,992,000	-34,752,000
Payable to customers	92,096,000	-46,877,000	0
Accrued compensation expense	-29,536,000	24,441,000	-18,494,000
Accrued expenses and other liabilities	-40,583,000	-64,470,000	-13,779,000
Net cash provided by (used in) operating activities	242,524,000	141,204,000	-64,516,000
Cash flows from investing activities
Proceeds and distributions from investments	58,660,000	3,251,000	72,196,000
Purchases of fixed assets and leasehold improvements	-34,139,000	-25,147,000	-31,389,000
Capitalized software development costs	-14,859,000	-2,556,000	0
Purchases of investments	-744,000	-158,000	-87,308,000
Investing activities from discontinued operations	0	12,963,000	0
Sale of trading rights	554,000	0	0
Net cash provided by (used in) investing activities	11,472,000	582,892,000	-46,501,000
Cash flows from financing activities
Proceeds from issuance of Credit Agreement	0	535,000,000	0
Partial repayment of Credit Agreement	-235,000,000	-300,000,000	0
Proceeds from issuance of Senior Secured Notes	0	305,000,000	0
Repayment of notes	0	-15,000,000	0
Payment of debt issuance cost	0	-34,592,000	-1,375,000
Issuance of equity to General Atlantic	0	55,000,000	0
Payment to former Knight Capital Group, Inc. stockholders	0	-720,000,000	0
Repayment of Knight Convertible Notes	0	-257,741,000	0
Funding of collateral account for Knight Convertible Notes	0	-117,259,000	0
Payment out of collateral account for Knight Convertible Notes	0	117,259,000	0
Borrowings under capital lease obligations	5,892,000	0	15,558,000
Principal payments on capital lease obligations	-9,232,000	-14,152,000	-20,990,000
Cost of common stock repurchased	-111,585,000	-11,324,000	0
Borrowings under secured credit facility	0	25,000,000	88,750,000
Repayment of secured credit facility	0	-25,000,000	-88,750,000
Members' distributions	0	-21,002,000	-37,061,000
Repurchase of members' interest	0	0	-25,173,000
Net cash used in financing activities	-349,925,000	-478,811,000	-69,041,000
Effect of exchange rate changes on cash and cash equivalents	416,000	1,365,000	0
(Decrease) increase in cash and cash equivalents	-95,513,000	246,650,000	-180,058,000
Cash and cash equivalents at beginning of period	674,281,000	427,631,000	607,689,000
Cash and cash equivalents at end of period	578,768,000	674,281,000	427,631,000
Supplemental disclosure of cash flow information:
Cash paid for interest	76,003,000	56,227,000	8,084,000
Cash paid for income taxes	16,975,000	10,198,000	21,789,000
Future Commission Merchant [Member]
Cash flows from investing activities
Cash received from sale of businesses	2,000,000	0	0
Knight Capital Group [Member]
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by operating activities
Non-cash gain on Knight Common Stock		-128,000,000
Cash flows from investing activities
Cash acquired upon acquisition of Knight Capital Group, Inc.	0	509,133,000	0
Urban Financial America [Member]
Cash flows from investing activities
Cash received from sale of businesses	$0	$85,406,000	$0

Organization_and_Description_o

Organization and Description of the Business	12 Months Ended
Organization and Description of the Business	Dec. 31, 2014
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.
	On December 19, 2012, Knight Capital Group, Inc.(“Knight”) and GETCO Holding Company, LLC (“GETCO”) entered into an agreement and plan of merger (as amended and restated on April 15, 2013, the “Merger Agreement”) for a series of strategic business combinations (the “Mergers”). The Mergers were approved by the respective stockholders and unitholders of both companies at special meetings held on June 25, 2013, and the Mergers were completed on July 1, 2013. As a result of the Mergers, Knight and GETCO each became a wholly-owned subsidiary of KCG.
	The Mergers took place in order to combine the businesses, intellectual capital and resources of the two companies to more successfully compete in the highly regulated and technologically advanced marketplace and to allow for further diversification of each company's revenues from principal and agency trading across asset classes and regions. The Mergers were treated as a purchase of Knight by GETCO for accounting and financial reporting purposes. As a result, the financial results for the year ended December 31, 2013 comprise six months of results of KCG plus six months of GETCO and the financial results for the year ended December 31, 2014 comprise the results of KCG. All periods prior to 2013 reflect solely the results and financial condition of GETCO.
	As of December 31, 2014, 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, foreign exchange, 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) an institutional spot foreign exchange electronic communication network ("ECN"); (iv) a fixed income ECN that also offers trading applications; and (v) 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 such as self-clearing services, including stock lending activities.
	Discontinued 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 October 2014, the Company announced that it began to explore strategic options for KCG Hotspot, the Company's spot institutional foreign exchange ECN, with the goal of executing on opportunities if it created additional value for the Company's stockholders, clients and employees.
	See Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale" and Footnote 27 "Subsequent Event" for additional information.

Merger_of_GETCO_and_Knight

Merger of GETCO and Knight	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Merger of GETCO and Knight	Merger of GETCO and Knight
	Background
	Pursuant to the Merger Agreement, each outstanding share of Knight Class A common stock, par value $0.01 per share (“Knight Common Stock”), was converted into the right to elect to receive either $3.75 per share in cash or one third of a share of KCG Class A common stock, par value $0.01 per share (“KCG Class A Common Stock”). As a result of the elections and proration procedures provided in the Merger Agreement, former Knight stockholders received cash payments aggregating $720.0 million and 41.9 million shares of KCG Class A Common Stock.
	Upon completion of the Mergers, GETCO unitholders received, in aggregate, 75.9 million shares of KCG Class A Common Stock and 24.3 million warrants to acquire shares of KCG Class A Common Stock. The warrants comprise 8.1 million Class A warrants, having a $12.00 exercise price and exercisable for a four-year term; 8.1 million Class B warrants, having a $13.50 exercise price and exercisable for a five-year term; and 8.1 million Class C warrants, having a $15.00 exercise price and exercisable for a six-year term (collectively the “KCG Warrants”).
	Accounting treatment of the Mergers
	The Mergers are accounted for as a purchase of Knight by GETCO under accounting principles generally accepted in the United States of America ("GAAP") based on, among other factors, the controlling ownership position of the former GETCO unitholders as of the closing of the Mergers. Under the purchase method of accounting, the assets and liabilities of Knight as of July 1, 2013 were recorded at their respective fair values and added to the carrying value of GETCO's existing assets and liabilities. The reported financial condition and results of operations of KCG for the periods following the Mergers reflect Knight's and GETCO's balances and reflect the impact of purchase accounting adjustments, including revised amortization and depreciation expense for acquired assets. As GETCO is the accounting acquirer, the financial results for KCG for the first six months of 2013 and the full year 2012 comprise solely the results of GETCO.
	Prior to the Mergers, GETCO treated its investment in Knight as an available-for-sale security, which it recorded at fair value, with any gains or losses recorded in other comprehensive income as a component of equity.
	All GETCO earnings per share and unit share outstanding amounts in these financial statements have been calculated as if the conversion of GETCO units to KCG Class A Common Stock took place on January 1, 2013, at the exchange ratio as defined in the Merger Agreement. See Footnote 20 "Earnings Per Share" for further information.
	Purchase price and goodwill
	The Knight acquisition was accounted for using the acquisition method of accounting. The aggregate purchase price of $1.37 billion was determined as the sum of the fair value of KCG shares issued to former Knight stockholders at closing; the fair value of Knight employee stock based awards attributable to periods prior to closing; and the fair value of the Knight Common Stock owned by GETCO and its subsidiaries immediately prior to the Mergers (and subsequently canceled in conjunction with the Mergers).
	The purchase price has been allocated to the assets acquired and liabilities assumed using their estimated fair values at July 1, 2013, the closing date of the Mergers. As of June 30, 2014, the Company, based on updated information, recorded purchase accounting adjustments that increased goodwill by $1.1 million, deferred tax assets by $3.1 million, and accrued expenses by $3.5 million, and decreased other assets by $0.7 million on the opening balance sheet at July 1, 2013 as well as the December 31, 2013 Consolidated Statement of Financial Condition. As of June 30, 2014, the Company had completed its analysis to finalize the allocation of the purchase price to the Knight acquired assets and liabilities.
	Tax treatment of the Mergers
	The Company believes that the Mergers will be treated as a transaction described in Section 351 of the Internal Revenue Code, and both Knight and GETCO have received tax opinions from external legal counsel to that effect. Knight’s tax basis in its assets and liabilities therefore generally carries over to the Company following the Mergers. Upon completion of the Mergers, the Company became a corporation subject to U.S. corporate income taxes and, following the Mergers, the Company recorded 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. The Company measures deferred taxes using the enacted tax rates and laws that will be in effect when such temporary differences are expected to reverse.
	The Company recorded net deferred tax assets of $65.5 million with respect to recording Knight’s assets and liabilities under the purchase method of accounting as described above as well as recording the value of tax net operating loss ("NOL”) carryforwards and other tax attributes acquired as a result of the Mergers, as described in Footnote 17 “Income Taxes”.
	The following table reflects the allocation of the purchase price to the assets acquired and liabilities assumed at the acquisition date (in thousands):


	Identifiable Net Assets
	Cash and cash equivalents				$	509,133

	Cash and cash equivalents segregated under federal and other regulations				203,045

	Financial instruments owned				1,937,929

	Securities borrowed				1,158,981

	Receivable from brokers, dealers and clearing organizations				1,369,474

	Fixed assets and leasehold improvements				80,280

	Investments				106,353

	Intangible assets				155,425

	Assets within discontinued operations				5,607,063

	Deferred tax asset, net				65,465

	Other assets				140,933

	Total Assets				$	11,334,081


	Financial instruments sold, not yet purchased				$	1,512,983

	Collateralized financings				1,166,211

	Payable to brokers, dealers and clearing organizations				635,914

	Payable to customers				527,918

	Accrued compensation expense				107,409

	Accrued expenses and other liabilities				130,010

	Liabilities within discontinued operations				5,518,168

	Debt				375,000

	Total Liabilities				$	9,973,613


	Total identified assets acquired, net of assumed liabilities				1,360,468


Goodwill				12,666


Total Purchase Price				$	1,373,134

Goodwill has been primarily assigned to the Market Making segment of the Company. None of the goodwill is expected to be deductible for tax purposes; however, as described in Tax treatment of the Mergers above, Knight’s tax basis in its assets, including certain goodwill, has carried over to the Company as a result of the Mergers.
Amounts allocated to intangible assets and goodwill, and the amortization period for intangible assets with finite useful lives, were as follows (dollars in thousands):


						Amortization
		Amount				Years
Technology		$	110,504			5 years

Customer relationships		35,000				9 - 11 years

Trade names		4,000				10 years

Trading rights (1)		5,921				7 years

Intangible assets		155,425

Goodwill		12,666

Total		$	168,091


-1	Trading rights include both assets with a finite useful life and assets with an indefinite useful life. The 7 years amortization period only applies to assets with a finite useful life.

Significant_Accounting_Policie

Significant Accounting Policies

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

Significant Accounting Policies

Basis of consolidation and form of presentation

The Consolidated Financial Statements, prepared in conformity with 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 11 "Investments", as a result of the merger of BATS Global Markets, Inc. ("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.

As a result of the change in accounting principle, the Consolidated Statement of Financial Condition at December 31, 2013 has been adjusted as follows: Investments increased by approximately $3.4 million, Deferred tax asset, net decreased by $1.1 million and Retained earnings increased by $2.3 million. The Consolidated Statements of Operations for the year ended December 31, 2013 have been adjusted to increase Investment income and other, net by $7.2 million and Net income by $6.1 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 includes commission equivalents earned on institutional client orders and commissions on futures transactions, and related expenses are also recorded on a trade date basis. 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 year ended December 31,

2014

2013

2012

Interest Income

14,363

10,187

3,070

Interest Expense

(13,742

)

(10,724

)

(5,427

)

Interest, net

621

(537

)

(2,357

)

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 year ended December 31,

2014

2013

2012

Dividend Income

45,910

26,237

2,253

Dividend Expense

(38,444

)

(21,630

)

(1,599

)

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 6 “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 either 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.

Prior to the Mergers, GETCO had a strategic investment in Knight which was classified as available for sale and held at fair value with any unrealized gains or losses recorded in Other comprehensive income or loss. As a result of the Mergers, the Company recognized a non-cash gain of $128.0 million on its investment in Knight Common Stock, which it recorded within Investment income and other, net on the Consolidated Statement of Operations for the year ended December 31, 2013 and reversed any previous unrealized gains out of Other comprehensive income.

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 the existence of an impairment. 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 this 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 years ended December 31, 2014, 2013 and 2012, the Company recorded a losses of $2.0 million, $4.1 million and a gain of $0.2 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. For qualifying net investment hedges, any gains or losses, to the extent effective, are included in Accumulated other comprehensive income on the Consolidated Statements of Financial Condition. The ineffective portion, if any, is recorded in Investment income and other, net on the Consolidated Statements of Operations.

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

Prior to the Mergers, GETCO and the majority of its subsidiaries were treated as partnerships or disregarded entities for U.S. income tax purposes and, accordingly, were not subject to federal income taxes. Instead, the former GETCO members were liable for federal income taxes on their proportionate share of taxable income. Upon completion of the Mergers, the Company became a corporation subject to U.S. corporate income taxes and, following the Mergers, 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.

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 March 2013, the Financial Accounting Standards Board (“FASB”) issued an Accounting Standard Update (“ASU”) concerning the parent's accounting for the cumulative translation adjustment upon derecognition of certain subsidiaries or groups of assets within a foreign entity or of an investment in a foreign entity. This ASU provides for the release of the cumulative translation adjustment into net income when a parent sells a part or all of its investment within a foreign entity, no longer holds a controlling interest in an investment in a foreign entity or obtains control of an investment in a foreign entity that was previously recognized as an equity method investment. This ASU was effective for reporting periods beginning after December 15, 2013. The adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.

In July 2013, the FASB issued an ASU to clarify the financial statement presentation of an unrecognized tax benefit when a NOL carryforward, a similar tax loss, or a tax credit carryforward exists. This ASU requires entities to present an unrecognized tax benefit as a reduction of a deferred tax asset for a NOL carryforward whenever the NOL or tax credit carryforward would be available to reduce the additional taxable income or tax due if the tax position is disallowed. The ASU was effective for reporting periods beginning after December 15, 2013. The adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.

In April 2014, the FASB issued an ASU that amends the requirements for reporting discontinued operations. Under the new guidance, discontinued operations reporting will be 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.

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 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 guidance is effective for reporting periods beginning after December 15, 2014. Early adoption is not permitted. 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 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.

Discontinued_Operations_Assets

Discontinued Operations & Assets and LIabilities Held for Sale	12 Months Ended
	Dec. 31, 2014
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations & Assets and LIabilities Held for Sale	Discontinued Operations & Assets and Liabilities Held for Sale
	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 the fourth quarter of 2013, and, as a result, the revenues and expenses of Urban's operations and costs of the related sale have been included in (Loss) Income from discontinued operations, net of tax within the Consolidated Statements of Operations for the six months ended December 31, 2013 and the year ended December 31, 2014.
	The revenues and results of operations of discontinued operations are summarized as follows (in thousands):


		For the year ended December 31, 2014			For the six months ended December 31, 2013
	Revenues & Gain on sale	$	(1,148	)	$	39,868


	Expenses:
	Compensation	$	70		$	14,068

	Payments for order flow	—			9,885

	Execution and clearance fees	—			5,038

	Other expenses	930			10,748

	Total Expenses	1,000			39,739

	Pre-tax (loss) income from discontinued operations	(2,148		)	129

	Income tax benefit (expense)	816			(49		)

	Loss (Income) from discontinued operations, net of tax	$	(1,332	)	$	80

	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 are included in the Global Execution Services segment and in Continuing Operations on the Consolidated Statements of Operations for all applicable years presented.
	In October 2014, the Company announced that it began to explore strategic options for KCG Hotspot, with the goal of executing on opportunities if it creates additional value for its stockholders, clients and employees. KCG Hotspot is a single disposal group that is considered to be held-for-sale as of December 31, 2014 and, as a result, certain assets and liabilities related to KCG Hotspot have been 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 believes that no impairment exists as the fair value of the net assets related to KCG Hotspot less the costs to sell the business will exceed the related carrying value at December 31, 2014. The Company does not believe that this sale represents a strategic shift that will have a major effect on its operations and financial results, and therefore KCG Hotspot will 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 years presented.
	The KCG Hotspot assets and liabilities held for sale 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

	In January 2015, KCG entered into an agreement to sell KCG Hotspot to BATS. See Footnote 27 “Subsequent Event” for additional information.

Assets_Segregated_or_Held_in_S

Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations	12 Months Ended
	Dec. 31, 2014
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 ("Segregated assets") primarily relate to the Company’s regulated businesses and consist of the following (in thousands):


		December 31,		31-Dec-13
		2014
	Cash and cash equivalents segregated under federal or other regulations	$	3,361	$	183,082

	Receivable from brokers, dealers and clearing organizations(1)	—		304,294

	Total assets segregated or held in separate accounts under federal or other regulations	$	3,361	$	487,376

	(1) Segregated assets included within Receivable from brokers, dealers and clearing organizations comprise cash and cash equivalents and U.S. government obligations primarily held as deposits with exchange clearing organizations.
	Segregated assets at December 31, 2013 primarily comprise assets related to the Company's FCM business that was sold during the fourth quarter of 2014.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments
	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 3 “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
	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

	Deferred compensation investments (3)	—			868			—			868

	Investment in Deephaven Funds (3)	—			146			—			146

	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, Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition.



		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	December 31, 2013	Level 1			Level 2			Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,298,785		$	—		$	—		$	2,298,785

	Listed options	339,798			—			—			339,798

	U.S. government and Non-U.S. government obligations	40,053			—			—			40,053

	Corporate debt	43,203			—			—			43,203

Total Financial instruments owned, at fair value	2,721,839			—			—			2,721,839

Securities on deposit with clearing organizations (2)	170,235			—			—			170,235

Investment in CME Group (3)	3,925			—			—			3,925

Deferred compensation investments (3)	—			117			—			117

Investment in Deephaven Funds (3)	—			1,958			—			1,958

Total assets held at fair value	$	2,895,999		$	2,075		$	—		$	2,898,074

Liabilities
Financial instruments sold, not yet purchased, at fair value:
Equities (1)	$	1,851,006		$	—		$	—		$	1,851,006

Listed options	252,282			—			—			252,282

U.S. government obligations	15,076			—			—			15,076

Corporate debt	42,122			—			—			42,122

Foreign currency forward contracts	—			5,014			—			5,014

Total liabilities held at fair value	$	2,160,486		$	5,014		$	—		$	2,165,500

(1) Equities of $697.9 million have been netted by their respective long and short positions by CUSIP number.

(2)	Securities on deposit with clearing organizations consist of U.S. government obligations and are recorded within Receivable from brokers, dealers and clearing organizations on the Consolidated Statements of Financial Condition.
(3) Investment in CME Group, Deferred compensation investments and Investment in Deephaven Funds 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 December 31, 2014 and December 31, 2013 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 12 “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, deferred compensation investments and remaining investment in the Deephaven Funds 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 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 for the year ended December 31, 2014.
At December 31, 2013, the Company had foreign currency contracts with a notional value of 80.0 million British Pounds. These forward contracts were used to hedge the Company's investment in its European subsidiaries. These forward contracts expired in the first quarter of 2014 and were not renewed.
The fair value of these forward contracts were determined based upon spot foreign exchange rates, LIBOR interest rates and dealer quotations.
Deferred compensation investments
Deferred compensation investments 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.
Investment in Deephaven Funds
Investment in Deephaven Funds represents the Company's residual investment in certain funds that were formerly managed by Deephaven Capital Management, a former Knight subsidiary. These investments are in the process of liquidation and are valued based upon the fair value of the underlying investments within such funds.

Derivative_Financial_Instrumen

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2014
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 December 31, 2014 and 2013 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-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 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		15,245			8,894			14,847			9,105

	Total			$	169,423		458,242			$	139,811		469,292



				31-Dec-13
		Financial Statements		Assets						Liabilities
		Location		Fair Value			Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	89		892			$	142		533

	Forward contracts	Other assets		6,913			1			6,501			1

	Forward contracts(1)	Financial instruments sold, not yet purchased, at fair value		—			—			5,014			1

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		223			1,069			1,089			1,046

	Swap contracts	Receivable from/Payable to brokers, dealers and clearing organizations		—			—			18			1

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		339,798			730,020			252,282			755,947

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		4,815			18,280			2,259			15,202

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		3,392			10,629			1,773			3,806

	Total			$	355,230		760,891			$	269,078		776,537


	(1)	Designated as hedging instrument.


					Gain (Loss) Recognized
			Financial Statements		For the year ended December 31,
		Location		2014			2013				2012
Derivative instruments not designated as hedging instruments:
Foreign currency
Futures contracts		Trading revenues, net		$	10,535		$	12,191			$	19,699

Forward contracts		Investment Income and other, net		526			—				—

Equity
Futures contracts		Trading revenues, net		25,247			50,073				76,089

Swap contracts		Trading revenues, net		5,277			11,736				8,116

Listed options (1)		Trading revenues, net		(37,439		)	37,035				50,446

Fixed income
Futures contracts		Trading revenues, net		31,277			80,511				81,933

Commodity
Futures contracts		Trading revenues, net		55,295			62,215				35,952

				$	90,718		$	253,761			$	272,235

Derivative instruments designated as hedging instruments:
Foreign exchange - forward contract		Accumulated other comprehensive income		$	—		$	(3,298	)		$	—


(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.
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_an

Collateralized Transactions and Assets and Liabilities Subject to Netting	12 Months Ended
	Dec. 31, 2014
Collateralized Agreements [Abstract]
Collateralized Transactions and Assets and Liabilities Subject to Netting	Collateralized Transactions and Assets and Liabilities Subject to Netting
	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):


		December 31,			December 31,
		2014			2013
	Collateral permitted to be delivered or repledged	$	1,586,700		$	1,315,803

	Collateral that was delivered or repledged	1,485,267			1,231,468

	Collateral permitted to be further repledged by the receiving counterparty	147,696			142,938

	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):


		December 31,			December 31,
		2014			2013
	Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge	$	536,124		$	552,242

	Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	979,652			676,956

	Assets and Liabilities Subject to Netting
	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). A master agreement creates a single contract under which all transactions between two counterparties are executed allowing for trade aggregation and a single net payment obligation. 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):


		December 31, 2014	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

		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

		Foreign currency forward contracts	60			—			60			—			—			60

		Swaps	85			—			85			—			—			85

		Futures	24,680			24,436			244			—			—			244

		Total Assets	$	1,823,018		$	24,436		$	1,798,582		$	1,591,619		$	15,782		$	191,181

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

		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

		Futures	24,436			24,436			—			—			—			—

		Total Liabilities	$	1,781,118		$	24,436		$	1,756,682		$	1,615,949		$	33,141		$	107,592

	(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 avalable 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 reverse repurchase agreements at broker dealer.


	31-Dec-13	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	$	339,798		$	—		$	339,798		$	—		$	—		$	339,798

	Securities borrowed	1,357,387			—			1,357,387			1,316,110			10,110			31,167

	Receivable from brokers, dealers and clearing organizations (3)	24,366			—			24,366			24,249			—			117

	Foreign currency forward contracts	6,913			6,501			412			—			—			412

	Futures	8,519			4,369			4,150			—			—			4,150

	Total Assets	$	1,736,983		$	10,870		$	1,726,113		$	1,340,359		$	10,110		$	375,644

	Liabilities
	Listed options	$	252,282		$	—		$	252,282		$	—		$	10,924		$	241,358

	Securities loaned	733,230			—			733,230			716,838			10,110			6,282

	Financial instruments sold under agreements to repurchase	640,950			—			640,950			640,948			—			2

	Foreign currency forward contracts	11,515			6,501			5,014			—			—			5,014

	Futures	5,263			5,263			—			—			—			—

	Swaps	18			—			18			—			—			18

	Total Liabilities	$	1,643,258		$	11,764		$	1,631,494		$	1,357,786		$	21,034		$	252,674

(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 avalable 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 reverse repurchase agreements at broker dealer.

Receivable_from_and_Payable_to

Receivable from and Payable to Brokers, Dealers and Clearing Organizations	12 Months Ended
	Dec. 31, 2014
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):


		December 31,		December 31, 2013
		2014
	Receivable:
	Clearing organizations and other	$	1,095,025	$	750,440

	Assets segregated or held in separate accounts under federal or other regulations (1)	—		304,294

	Securities failed to deliver	93,808		202,517

	Total Receivable	$	1,188,833	$	1,257,251

	Payable:
	Clearing organizations and other	$	350,627	$	425,196

	Securities failed to receive	325,462		48,912

	Total Payable	$	676,089	$	474,108

	(1) Segregated assets primarily related to the FCM Business, which was sold in the fourth quarter of 2014, included within Receivable from brokers, dealers and clearing organizations comprise cash and cash equivalents and U.S. government obligations primarily held as deposits with exchange clearing organizations.
	Segregated assets at December 31, 2013 primarily comprise assets related to the Company's FCM business that was sold during the fourth quarter of 2014.
	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.

Fixed_Assets_and_Leasehold_Imp

Fixed Assets and Leasehold Improvements	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Fixed Assets and Leasehold Improvements	Fixed Assets and Leasehold Improvements
	Fixed assets and leasehold improvements comprise the following (in thousands):


		Depreciation	31-Dec-14			31-Dec-13
		Period
	Computer hardware and software	3 years	$	242,498		$	223,141

	Leasehold improvements	*	114,851			115,046

	Telephone systems and equipment	5 years	3,962			3,848

	Furniture and fixtures	7 years	12,120			12,227

			373,431			354,262

	Less - Accumulated depreciation and amortization		(239,380		)	(207,594		)
			$	134,051		$	146,668

	*Shorter of life of lease or useful life of assets

Investments

Investments	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Investments	Investments
	Investments comprise strategic investments, deferred compensation investments, and investment in the Deephaven Funds. Investments consist of the following (in thousands):


		December 31,		December 31,
		2014		2013
	Strategic investments:
	Investments accounted for under the equity method	$	86,328	$	110,460

	Investments held at fair value	4,435		3,925

	Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	8,949		8,953

	Total Strategic investments	99,712		123,338

	Deferred compensation investments	868		117

	Investment in Deephaven Funds	146		1,958

	Total Investments	$	100,726	$	125,413

	Investments held at fair value are accounted for as available for sale securities and any unrealized gains or losses are recorded 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, completed their previously announced merger into BATS. 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 a board seat. Based on these facts, the Company will account for its interest in BATS under the equity method. This change in accounting principle is applied retrospectively and, as such, the Company has presented the adjustment as a prior period adjustment to the 2013 consolidated results of operations. See Footnote 3 "Significant Accounting Policies" for a discussion of the effect of this retrospective adjustment.
	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 respective earnings of $6.6 million.
	tradeMONSTER Group, Inc.
	Prior to August 2014, the Company held an investment in tradeMONSTER Group, Inc. ("tradeMONSTER") which it accounted for under the equity method of accounting. In August 2014, tradeMONSTER combined with OptionsHouse LLC ("OptionsHouse") to form TM Holdings, L.P. (“TM Holdings”). The Company will continue to account for its interest in TM Holdings under the equity method of accounting.
	During the third quarter of 2014 the Company recognized a net gain of $15.1 million related to the combination, which is recorded within Investment income and other, net in the Consolidated Statements of Operations. The net gain of $15.1 million comprises a gain on the Company's exchange of its investment in tradeMONSTER for its investment in TM Holdings of $17.6 million offset, in part, by the Company's share of tradeMONSTER’s transaction costs.

Goodwill_and_Intangible_Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2014
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.
	As discussed in Footnote 2 "Merger of GETCO and Knight", as a result of the Mergers, $155.4 million and $12.7 million in identifiable intangible assets and goodwill, respectively, were recorded by the Company as of the date of the Mergers.
	No events occurred in the years ended December 31, 2014 or 2013 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 and 2013, 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):


		December 31,				December 31, 2013
		2014
	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 9 years. The weighted average remaining life of the Company’s intangible assets with definite useful lives at December 31, 2014 and December 31, 2013 was approximately five and six years, respectively.
	The following tables summarize the Company’s Intangible assets, net of accumulated amortization by segment and type (in thousands):


		December 31,				31-Dec-13
		2014
	Market Making
	Technology	$	50,542			$	53,315

	Trading rights	44,358				48,920

	Total	94,900				102,235

	Global Execution Services (1)
	Technology	18,200				38,682

	Customer relationships	10,833				33,278

	Trade names	850				3,800

	Total	29,883				75,760

	Corporate and Other
	Technology	10,500				13,500

	Total	$	135,283			$	191,495


	(1)	Excluded from the December 31, 2014 balance is $34.7 million of intangibles related to the KCG Hotspot which is held for sale. As noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", such amount is included in Assets of business held for sale at December 31, 2014.


			December 31,				31-Dec-13
			2014
	Technology (1)	Gross carrying amount	$	115,804			$	120,346

		Accumulated amortization	(36,562		)		(14,849		)
		Net carrying amount	79,242				105,497

	Trading rights (2)	Gross carrying amount	62,468				62,450

		Accumulated amortization	(18,110		)		(13,530		)
		Net carrying amount	44,358				48,920

	Customer relationships (3)	Gross carrying amount	13,000				35,000

		Accumulated amortization	(2,167		)		(1,722		)
		Net carrying amount	10,833				33,278

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

	Accumulated amortization	(150		)		(200		)
	Net carrying amount	850				3,800

Total	Gross carrying amount	192,272				221,796

	Accumulated amortization	(56,989		)		(30,301		)
	Net carrying amount	$	135,283			$	191,495


(1)	The weighted average remaining life for technology, including capitalized software, was approximately 3 and 4 years as of December 31, 2014 and December 31, 2013, respectively. Excluded from the December 31, 2014 balance is $13.1 million of technology assets related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", 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 and 8 years as of December 31, 2014 and December 31, 2013, respectively. As of December 31, 2014 and December 31, 2013, $6.8 million and $7.6 million, respectively, of trading rights had indefinite useful lives.

(3)	Customer relationships relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 8 and 10 years as of December 31, 2014 and December 31, 2013, 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 the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", is included in Assets of business held for sale at December 31, 2014.

(4)	Trade names relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 9 and 10 years as of December 31, 2014 and December 31, 2013, respectively. Excluded from the December 31, 2014 balance is $2.6 million of trade name related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", 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 year ended December 31,
	2014				2013				2012
Amortization expense	$	35,592			$	19,211			$	5,518

As of December 31, 2014, the following table summarizes the Company’s estimated amortization expense for future periods (in thousands):


	Amortization
	expense
For the year ended December 31, 2015	$	31,180

For the year ended December 31, 2016	30,049

For the year ended December 31, 2017	27,244

For the year ended December 31, 2018	15,611

For the year ended December 31, 2019	6,211

Debt

Debt	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Debt	Debt
	The carrying value and fair value of the Company's debt is as follows (in thousands):


		December 31, 2014							December 31, 2013
		Carrying Amount				Fair Value			Carrying Amount		Fair Value
	Cash Convertible Senior Subordinated Notes	$	117,259			$	116,819		$	117,259	$	118,432

	Senior Secured Notes	305,000				309,194			305,000		320,823

	First Lien Credit Facility	—				—			235,000		235,000

	Total Debt	$	422,259			$	426,013		$	657,259	$	674,255

	The fair value of the Company's debt is based upon the value of such debt in the secondary market. The carrying value of the First Lien Credit facility approximated fair value as it was not materially sensitive to shifts in interest rates due to its floating interest rate, which also considers changes in the Company's credit quality and financial condition. These liabilities would all 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 Convertible Notes bear 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 and will mature on March 15, 2015, subject to earlier repurchase or conversion. The Convertible Notes are reported as Debt in the Company’s Consolidated Statements of Financial Condition.
	In connection with the closing of the Mergers, on July 1, 2013, KCG became a party to the indenture under which Knight's $375.0 million Convertible Notes were issued. On July 1, 2013, the Company delivered a notice (the “Convertible Notes Notice”) to the holders of the Notes. The Convertible Notes Notice advised holders of the Convertible Notes of the following (among others):

	•	The completion of the Mergers on July 1, 2013 and the results of the election of the holders of Knight Common Stock to receive cash consideration for such Knight Common Stock constitutes a “Fundamental Change";

	•	Each holder of the Convertible Notes had the right to deliver a “Fundamental Change Repurchase Notice” requiring the Company to repurchase all or any portion of the principal amount of the Convertible Notes at a Fundamental Change Repurchase Price of 100% of the principal amount plus accrued and unpaid interest on August 5, 2013, the Fundamental Change Repurchase Date; and

	•	The Company deposited with the paying agent an amount of money sufficient to repurchase all of the Convertible Notes to be repurchased; and upon payment by the paying agent such Convertible Notes will cease to be outstanding.
	On July 1, 2013, $375.0 million, which was the amount needed to repurchase the aggregate amount of Convertible Notes in full at maturity, was deposited in a cash collateral account under the sole dominion and control of the paying agent under the First Lien Credit Facility (the "Collateral Account").
	After the Mergers, a total of $257.7 million in principal amount of the Convertible Notes were repurchased using funds deposited in the Collateral Account. The repurchase included accrued and unpaid interest of $3.6 million. In October 2013, after receiving consent from the Holders of the Senior Secured Notes (as defined below), the funds remaining in the Collateral Account were used to repay a portion of the First Lien Credit Facility. As of December 31, 2014 and December 31, 2013 there were no funds in the Collateral Account. As of December 31, 2014 and December 31, 2013, $117.3 million principal amount of the Convertible Notes were outstanding.
	The Convertible Notes are convertible based on a conversion rate, which was adjusted as a result of the Mergers, of 15.9728 shares of KCG common stock per $1,000 principal amount of Notes, which is equivalent to a conversion price of approximately $62.61 per share. The conversion rate and conversion price will be subject to further adjustment in certain events, such as distributions of dividends or stock splits.
	Upon any cash conversion, KCG will deliver an amount of cash calculated over the applicable 40-trading day observation period. KCG will not deliver its common stock (or any other securities) upon conversion under any circumstances. In addition, following certain corporate events that may occur prior to the maturity date, KCG is required to pay a cash make-whole premium by increasing the conversion rate for a holder that that elects to convert its Convertible Notes in connection with such a corporate event in certain circumstances. Subject to certain exceptions, holders may require KCG to repurchase, for cash, all or part of the Convertible Notes upon a “Fundamental Change” at a price equal to 100% of the principal amount of the Convertible Notes being repurchased plus any accrued and unpaid interest.
	The requirement that KCG settle conversions of the Convertible Notes entirely in cash gives rise to a bifurcatable derivative instrument under GAAP (the “embedded conversion derivative”). Based largely on the conversion price of the Convertible Notes compared to the price of KCG common stock on the relevant dates, the fair value of the embedded conversion derivative was determined to be $0 at December 31, 2014 and December 31, 2013.
	Debt incurred in connection with Mergers
	In connection with the Mergers, KCG entered into a series of debt financing transactions. Described below are the details of these transactions.
	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 “Senior Secured Notes”) pursuant to an indenture, dated June 5, 2013 (as amended and supplemented, the "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 Senior Secured Notes plus certain escrow agent fees and expenses of $3.0 million.
	On July 1, 2013, KCG and certain subsidiary guarantors (the "Guarantors") under the First Lien Credit Facility, as defined below, entered into a Second Supplemental Indenture, whereby the Senior Secured Notes and the obligations under the Senior Secured Notes Indenture will be fully and unconditionally guaranteed on a joint and several basis by the Guarantors and are secured by second-priority pledges and second-priority security interests in, and mortgages on, the collateral securing the First Lien Credit Facility, subject to certain exceptions. All of the subsidiary guarantors are wholly-owned subsidiaries of KCG.
	The Senior Secured Notes mature on June 15, 2018 and bear interest at a rate of 8.250% per year, payable on June 15 and December 15 of each year, beginning on December 15, 2013.
	On or after June 15, 2015, KCG may redeem all or a part of the 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 under the Senior Secured Notes Registration Rights Agreement (defined below), if any, on the Senior Secured Notes redeemed, to the applicable redemption date, if redeemed during the 12-month period beginning on June 15 of the years indicated below:


	Year		Percentage
	2015		104.125	%
	2016		102.063	%
	2017 and thereafter		100	%
	KCG may also redeem the 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 Senior Secured Notes to be redeemed, plus a make-whole premium and accrued and unpaid interest, if any. In addition, at any time on or prior to June 15, 2015, KCG may redeem up to 35% of the aggregate principal amount of the Senior Secured Notes with the net cash proceeds of certain equity offerings, at a price equal to 108.25% of the aggregate principal amount of the Senior Secured Notes, plus accrued and unpaid interest, if any.
	The Senior Secured Notes 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 December 31, 2014 and 2013, the Company was in compliance with the covenants.
	On July 1, 2013, KCG and the Guarantors entered into a joinder to the registration rights agreement dated June 5, 2013, (the "Senior Secured Notes Registration Rights Agreement") between Finance LLC and Jefferies LLC as representative of the initial purchasers of the Senior Secured Notes. Pursuant to the registration rights agreement, KCG shall use commercially reasonable efforts to (i) file an exchange offer registration statement with the SEC with respect to a registered offer to exchange the Senior Secured Notes (the "Exchange Offer"), (ii) issue in exchange for the Senior Secured Notes a new series of exchange notes within 365 days after June 5, 2013, and, (iii) in certain circumstances, file a shelf registration statement with respect to resales of the Senior Secured Notes. If KCG and the Guarantors fail to comply with certain obligations under the Senior Secured Notes Registration Rights Agreement, additional interest of up to 1.00% per annum will begin to accrue and be payable on the Senior Secured Notes.
	In October 2013, the Company received consents from holders ("Holders") of 99.7% of the aggregate principal amount of the Senior Secured Notes outstanding to amend, among other things, the terms of the Senior Secured Notes among the Company, The Bank of New York Mellon, as trustee and collateral agent (the “Trustee”), and the Guarantors. As a result, the Company entered into the Third Supplemental Indenture with the Trustee to amend the Senior Secured Notes Indenture to permit the purchase, redemption or repayment of the Convertible Notes at any price, including at a premium or at a discount from the face value thereof, with any available cash.
	In May 2014, the Company received consents from Holders of 98.5% of the aggregate principal amount of the Senior Secured Notes outstanding to amend the terms of the Senior Secured Notes Registration Rights Agreement. As a result, the Company entered into the First Amendment (the “Amendment”) to the Senior Secured Notes Registration Rights Agreement. The Amendment (i) postponed the deadline by which the Company must use commercially reasonable efforts to prepare and file the Exchange Offer with the SEC, from June 5, 2014 to June 30, 2015, and (ii) postponed the deadline by which the Company must use commercially reasonable efforts to file with and have declared effective by the SEC a shelf registration statement to cover certain resales of the Senior Secured Notes, from June 5, 2014 to June 30, 2015. The Amendment also had the effect of postponing the date on which an additional interest, which constitutes liquidated damages and is the exclusive remedy available to Holders for failing to register the Senior Secured Notes, begins to accrue as a result of failing to consummate the Exchange Offer or have a shelf registration statement declared effective. Accordingly, the Company and the Guarantors no longer have any obligation to pay Holders additional interest as of June 5, 2014, even though the Company and the Guarantors did not prepare, file or have declared effective a registration statement or a shelf registration statement or consummate the Exchange Offer by such date. As of the date of this filing, the Company has not registered the Senior Secured Notes and if it does not do so by June 30, 2015, additional interest will begin to accrue on the notes.
	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 conjunction with these payments, the Company wrote down $13.2 million of its capitalized debt costs associated with the Credit Agreement.
	In 2014, the Company repaid the remaining $235.0 million of principal of the First Lien Credit Facility out of available cash and the Credit Agreement was terminated. In conjunction with these payments, the Company wrote down the remaining $9.6 million of its capitalized debt costs associated with the Credit Agreement.
	The First Lien Credit Facility bore interest, at KCG's option, at a rate based on the prime rate (“First Lien Prime Rate Loans”) or based on LIBOR (“First Lien Eurodollar Loans”). First Lien Prime Rate Loans bore interest at a rate per annum equal to the greatest of prime rate, 2.25%, the federal funds rate plus 0.50%, and an adjusted one-month LIBOR rate plus 1.00%, in each case plus an applicable margin of 3.50%. First Lien Eurodollar Loans bore interest at a rate per annum equal to the adjusted LIBOR rate (subject to a 1.25% LIBOR floor) corresponding to the interest period plus an applicable margin of 4.50% per annum.
	The Company incurred issuance costs of $38.5 million in connection with the issuance of Senior Secured Notes, Credit Agreement and consent solicitations. The remaining issuance costs are recorded within Other assets on the Consolidated Statements of Financial Condition and are being amortized over the remaining term of the Senior Secured Notes. Including issuance costs, the Senior Secured Notes had an effective yield of 9.1%.
	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 December 31, 2014 and 2013, 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 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 recorded expenses with respect to the Debt as follows (in thousands):


	For the year ended December 31,
	2014				2013			2012
Interest expense	$	31,724			$	34,138		$	1,194

Amortization of debt issuance cost (1)	13,217				16,832			276

Commitment fee	1,575				792			1,034

Total	$	46,516			$	51,762		$	2,504


(1)	Of the $13.2 million of amortization of debt issuance cost incurred during 2014, $9.6 million is included in Writedown of debt issuance costs and $3.6 million is in Other expenses. The writedown amounts were incurred as a result of the $235.0 million repayment of the First Lien Credit Facility made in 2014. For 2013, $3.6 million is included in Other expense while $13.2 million is included in Writedown of debt issuance cost as a result of the $300.0 million repayment of the First Lien Credit Facility.

Related_Parties

Related Parties	12 Months Ended
	Dec. 31, 2014
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 activities with three investees whereby KCG accounts for its investment under the equity method of accounting. Each is considered a related party for all applicable periods.
	The Company earns revenues, incurs expenses and maintains balances with these 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 year ended December 31,
	Statements of Operations	2014			2013
	Revenues
	Commissions and fees	$	14,528		$	4,045

	Trading revenues, net	3,862			437

	Interest, net	651			71

	Total revenues from related parties	$	19,041		$	4,553

	Expenses
	Execution and clearance fees(1)	$	(10,261	)	$	(19,491	)
	Payment for order flow	585			—

	Collateralized financing interest	529			102

	Other expense	1,719			651

	Total expenses incurred with respect to related parties	$	(7,428	)	$	(18,738	)
	(1) Represents net volume based fees received from providing liquidity to related trading venues.


	Statements of Financial Condition	December 31,			December 31,
		2014			2013
	Assets
	Securities borrowed	$	26,110		$	57,732

	Receivable from brokers, dealers and clearing organizations	20,075			20,826

	Other assets	—			277

	Liabilities
	Securities loaned	$	7,376		$	116,062

	Payable to brokers, dealers and clearing organizations	8,509			17,911

	Accrued expenses and other liabilities	5,667			179

	In the ordinary course of business, the Company enters into foreign exchange contracts with related parties.
	During 2013, the Company paid one of the related parties $49.8 million in fees related to financing and advisory activities.

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2014
Share-based Compensation [Abstract]
Stock-Based Compensation	Stock-Based Compensation
	KCG Equity Incentive Plan
	The Knight Capital Group, Inc. Amended and Restated 2010 Equity Incentive Plan was established to provide long-term incentive compensation to employees and directors of the Company. As a result of the Mergers, on July 1, 2013, this plan was assumed by KCG and was renamed the KCG Holdings, Inc. Amended and Restated Equity Incentive Plan (the "KCG Plan"). As of December 31, 2014, there were approximately 30.1 million shares authorized for issuance under the KCG Plan, of which approximately 16.3 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, and allows for the grant of 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. The KCG Plan replaced prior Knight stockholder-approved equity plans for future equity grants and no additional grants will be made under those historical Knight stock plans. However, the terms and conditions of any outstanding equity grants under the historical Knight stock plans are not affected.
	As a result of the Mergers on July 1, 2013, each outstanding Knight stock option, whether vested or unvested, was automatically replaced with an option to purchase KCG Class A Common Stock equal to one third of the number of shares of Knight Common Stock subject to such original stock option immediately prior to the completion of the Mergers (rounded down to the nearest whole share of KCG Class A Common Stock). The exercise price per share of KCG Class A Common Stock is equal to the exercise price per share of Knight Common Stock subject to such Company stock option multiplied by three (rounded up to the nearest whole cent). Pursuant to the terms of the applicable Knight stock plans and award agreements, each option granted on or prior to December 19, 2012 immediately vested. There were no Knight stock options granted subsequent to December 19, 2012.
	As a result of the Mergers, each outstanding Knight restricted share and Knight RSU granted after December 19, 2012 was replaced with a KCG restricted share or RSU, as applicable, in respect of one third of a share of common stock of KCG (rounded to the nearest whole share). Knight restricted share and RSU awards granted on or prior to December 19, 2012 (except for RSUs subject to performance-based vesting conditions) automatically vested upon the completion of the Mergers. Knight restricted share and RSU awards granted after December 19, 2012 (and RSUs granted on or prior to December 19, 2012 that were subject to performance-based vesting conditions) continue to vest in accordance with their existing vesting schedule, subject to acceleration under certain circumstances.
	Restricted Shares and Restricted Stock Units
	Eligible employees and directors may receive restricted shares and/or RSUs (collectively “restricted awards”) as a portion of their total compensation. The majority of restricted awards vest ratably over three years and are subject to accelerated vesting, or continued vesting, following certain termination circumstances, in accordance with the applicable award documents and employment agreements between the Company and the participant. For certain restricted awards, the Company has the right to fully vest employees and directors upon retirement and in certain other circumstances.
	The Company measures compensation cost related to restricted awards based on the fair value of KCG Class A Common Stock at the date of grant. Compensation expense from continuing operations relating to restricted awards, which is primarily recorded in Employee compensation and benefits, and the corresponding income tax benefit, which is recorded in Income tax (benefit) expense on the Consolidated Statements of Operations are presented in the following table (in thousands):


		For the year ended December 31,
		2014				2013
	Stock award compensation expense	$	55,402			$	33,067

	Income tax benefit	21,053				11,573

	The following table summarizes restricted awards activity for the year ended December 31, 2014 (awards in thousands):


			Restricted Stock Units
			Number of			Weighted-
			Units			Average
						Grant date
						Fair Value
	Outstanding at December 31, 2013		8,420			$	10.71

	Granted		4,937			10.99

	Vested		(3,464	)		10.77

	Forfeited		(746	)		11.66

	Outstanding at December 31, 2014		9,147			$	10.77

	There is $54.8 million of unamortized compensation related to unvested RSUs outstanding at December 31, 2014. The cost of these unvested RSUs is expected to be recognized over a weighted average life of 1.6 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, in each case with an exercise price not less than the market value of KCG Class A Common Stock on the grant date. Options and SARs 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. 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. Options and SARs are otherwise canceled if employment is terminated before the end of the relevant vesting period. The Company’s policy is to issue new shares upon option exercises by its employees and directors. The Company may issue new shares or provide a cash payment upon SARs exercises by its employees.
	The fair value of each option and SAR granted is estimated as of its respective grant date using the Black-Scholes option-pricing model. Stock options and SARs are granted with exercise prices equal to or greater than the market value of the Company’s common stock at the date of grant as defined by the KCG Plan. The principal assumptions utilized in valuing options and SARs and the methodology for estimating such model inputs include: 1) risk-free interest rate—estimate is based on the yield of U.S. zero coupon securities with a maturity equal to the expected life of the option or SAR; 2) expected volatility—estimate is based on several factors including implied volatility of market-traded 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—estimate is based on internal studies of historical experience and projected exercise behavior based on different employee groups and specific option and SAR characteristics, including the effect of employee terminations. There were no stock options or SARs granted during the year ended December 31, 2014.
	The weighted average assumptions used for stock options granted in 2013 were as follows:


		2013
	Dividend yield	—	%
	Expected volatility	35	%
	Risk-free interest rate	1	%
	Expected life (in years)	3.5

	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 (benefit) expense on the Consolidated Statements of Operations are as follows (in thousands):


		For the year ended December 31,
		2014				2013
	Stock option and SAR compensation expense	$	3,807			$	1,813

	Income tax benefit	1,447				635

	The following table summarizes stock option and SAR activity and stock options exercisable for the year ended December 31, 2014 (awards in thousands):


			Number of Stock Awards			Weighted-				Aggregate			Weighted-
						Average				Intrinsic			Average
						Exercise				Value			Remaining
						Price							Life (years)
Outstanding at December 31, 2013 (1)		4,967			$	18.45

Granted at market value		—			—

Exercised		—			—

Forfeited or expired		(276	)		36.16

Outstanding at December 31, 2014 (1)		4,691			$	17.41			$	3,438		3.41

Exercisable at December 31, 2014		1,809			$	20.95			$	1,146		3.23

Available for future grants at December 31, 2014 (2)		16,291

(1) Includes 1.7 million of SARs.
(2) Represents options, SARs and awards available for grant under the KCG Plan.


		Options Outstanding									Options Exercisable
Range of Exercise Prices		Outstanding			Weighted-		Weighted-				Number		Weighted-
		at 12/31/14			Average		Average				Exercisable		Average
					Remaining		Exercise				at 12/31/14		Exercise
					Contractual		Price						Price
					Life
$8.24 - $8.24		833			3.61		$	8.24			278		$	8.24

$8.25- $8.25		91			3.64		8.25				30		8.25

$11.65 - $11.65		1,700			3.51		11.65				567		11.65

$22.50 - $22.50		1,700			3.51		22.5				567		22.5

$23.70 - $52.98		294			2.01		40.94				294		40.94

$53.91 - $53.91		74			2.08		53.91				74		53.91

		4,691			3.41		$	17.41			1,809		$	20.95

The aggregate intrinsic value is the amount by which the closing price of the Company’s 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 year ended December 31, 2014.
There is $3.3 million of unamortized compensation related to unvested stock options and SARs outstanding at December 31, 2014. The cost of these unvested awards is expected to be recognized over a weighted average life of 0.9 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 KCG Class A Common Stock. The units are marked to the current stock price of KCG 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 units as of June 30, 2013. Deferred compensation payable at December 31, 2014 and December 31, 2013 related to incentive units was $2.9 million and $3.8 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 year ended December 31, 2014 (units in thousands):


	Vested
Incentive units at December 31, 2013	49

Issued	—

Vested	—

Exercised	(11	)
Canceled	—

Incentive units at December 31, 2014	38

Class B units
Prior to the Mergers, GETCO granted membership unit awards to employees in the form of Class B units. The Class B units were valued based on the same methodology used to value the GETCO incentive units. Prior to 2012, these non-voting units vested in full three years from the grant date, provided certain conditions of employment and performance were met by the employee. In 2012, GETCO changed the vesting of units granted in 2012 to annual vesting of one-third of the units over a three year period. Upon termination of employment, GETCO had the option to repurchase all or a portion of the units granted within six months. The purchase price for the unvested units was determined as a percentage of grant date fair value. GETCO classified these unit awards as equity as the employees received full membership rights with respect to allocation of income and participation in member distributions. In connection with the Mergers, all outstanding unvested Class B units vested on June 25, 2013.
Class E units
In 2012, GETCO also granted employees profit interests in the form of Class E units. Prior to 2012, Class E units primarily vested in full three years from the grant date. For units granted in 2012, GETCO changed the vesting of Class E units to an annual vesting of one-third of the units over the three year period and provided GETCO an option to repurchase the units at the end of 5 years. Class E units allowed for future appreciation in excess of the GETCO's value over a certain strike price per unit and allocation of income once the units are vested. Upon the departure of an employee, the Class E units were forfeited whether vested or not, and if vested, the cash value of the Class E units above their strike price was paid to the employee. GETCO classified these unit awards as equity. In connection with the Mergers all outstanding unvested Class E units vested on June 25, 2013 and were canceled for no consideration.
As noted, in connection with the Mergers, all outstanding incentive units, Class B units and Class E units vested, and as a result the remaining unamortized expense was accelerated. The accelerated amortization expense recorded during the year ended December 31, 2013 for incentive units, Class B units and Class E units was $1.3 million, $9.4 million and $3.5 million, respectively.
Compensation expense (benefit) related to the Class B, Class E and Incentive units, all of which are recorded within Employee compensation and benefits on the Consolidated Statements of Operations are as follows (in thousands):


	For the year ended December 31,
	2014				2013				2012
Class B and E units	$	—			$	19,860			$	12,320

Incentive units	(269		)		2,446				1,264

Total	$	(269	)		$	22,306			$	13,584

Employee_Benefit_Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Employee Benefit Plan	Employee Benefit Plan
	The Company sponsors a 401(k) profit sharing plan (the “401(k) Plan”) in which most of its employees are eligible to participate. Under the terms of the 401(k) Plan, the Company is required to make annual contributions to the 401(k) Plan equal to 100% of the contributions made by its employees, up to annual limits, however for 2013, such provisions only applied to Knight employees. The total expense recognized with respect to the 401(k) Plan is included in Employee compensation and benefits on the Consolidated Statements of Operations, as follows (in thousands):


		For the year ended December 31,
		2014		2013
	Total expense	$	10,093	$	2,959

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	Upon completion of the Mergers, the Company became subject to U.S. corporate income taxes. The Company and its subsidiaries 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.
	Prior to the Mergers, GETCO and the majority of its subsidiaries were treated as partnerships or disregarded entities for U.S. income tax purposes and, accordingly, were not subject to federal income taxes. Instead, former GETCO members were liable for federal income taxes on their proportionate share of taxable income; however, certain subsidiaries were subject to corporate income taxes related to the taxable income generated by their operations.
	As described in Footnote 2 “Merger of GETCO and Knight”, following the Mergers on July 1, 2013 the Company recorded $65.5 million of deferred tax assets as a result of recording Knight’s assets and liabilities under the purchase method of accounting as well as recording the value of Knight’s NOLs and tax credit carryforwards as described below.
	As a result of the Company becoming subject to U.S. corporate income taxes, the Company also recorded, upon the closing of the Mergers on July 1, 2013, a nonrecurring $103.5 million deferred tax benefit and corresponding deferred tax asset relating to GETCO's existing tax attributes. This deferred tax asset primarily relates to differences between GETCO’s book and tax bases in its intangible assets and its strategic investments.
	The provision (benefit) for income taxes from continuing operations consists of (in thousands):


		For the year ended December 31,
		2014				2013			2012
	Current:
	U.S. federal	$	709			$	(194	)	$	(627	)

	U.S. state and local	4,081				2,902			618

	Non U.S.	(828		)		3,773			8,708

		$	3,962			$	6,481		$	8,699

	Deferred:
	U.S. federal	$	30,331			(106,996		)	1,771

	U.S. state and local	(10,997		)		—			1,046

	Non U.S.	(543		)		(599		)	(1,240		)
		$	18,791			$	(107,595	)	$	1,577


	Provision (benefit) for income taxes	$	22,753			$	(101,114	)	$	10,276


	The following table reconciles the U.S. federal statutory income tax to the Company's actual income tax from continuing operations (in thousands):


		For the year ended December 31,
		2014				2013			2014

	U.S. federal statutory income tax expense	$	29,815			$	8,742		$	9,249

	Income not subject to U.S. corporate income tax	—				(15,583		)	(7,095		)

	U.S. state and local income taxes, net of U.S. federal income tax effect	(4,495		)		1,881			772

	Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	—				(103,499		)	—

	Nondeductible expenses (1)	230				3,627			200

	Foreign taxes	(1,371		)		3,603			7,468

	Federal research & development tax credits	(1,241		)		—			—

	Other, net	(185		)		115			(318		)

	Income tax expense (benefit)	$	22,753			$	(101,114	)	$	10,276

	(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.
	The Company’s net deferred tax assets are reported as Deferred tax asset, net on the Consolidated Statements of Financial Condition. At December 31, 2014, and December 31, 2013, the Company’s net deferred tax assets were $154.8 million and $175.6 million, respectively, and comprised the following:


		For the year ended December 31,
		2014				2013
	Deferred tax assets:
	Employee compensation and benefit plans	$	24,587			$	16,629

Fixed assets and other amortizable assets	82,882				90,130

Reserves	2,668				5,589

Valuation of investments	32,962				30,073

Net operating loss carryforwards, net	94,770				127,637

Less: Valuation allowance on net operating loss carryforwards	(15,238		)		(36,043		)
Total deferred tax assets	$	222,631			$	234,015


Deferred tax liabilities
Employee compensation and benefit plans	$	—			$	—

Fixed assets and other amortizable assets	26,244				29,077

Reserves	2,226				—

Valuation of investments	19,912				10,414

Reduction in foreign tax credit for Non-U.S. NOL carryforwards	19,490				18,885

Total deferred tax liabilities	67,872				58,376

Net deferred tax assets	$	154,759			$	175,639

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 its federal deferred tax assets in the future. With the exception of certain NOLs discussed below, the Company has not recorded any valuation allowance with respect to its federal deferred tax assets at December 31, 2014 or December 31, 2013. Management believes that positive evidence including the Company's history of sustainable profitability and its forecasts of future profitability outweighs the negative evidence. The Company has also 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.
At December 31, 2014 and December 31, 2013, the Company had total U.S. federal NOL carryforwards of $133.5 million and $194.5 million, respectively, of which $28.5 million and $89.0 million, respectively, resulted from the acquisition of Knight. The Company recorded a related deferred income tax asset related to these federal NOLs of $46.7 million and $68.1 million at December 31, 2014 and December 31, 2013, respectively, and a partially offsetting valuation allowance of $6.8 million at each December 31, 2014 and December 31, 2013, which represents the portion of these net operating loss carryforwards that are considered more likely than not to expire unutilized.
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 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 December 31, 2014 and December 31, 2013, the Company had, in multiple jurisdictions, aggregate state and local NOL carryforwards of $1.28 billion and $1.66 billion, respectively, all of which resulted from the acquisition of Knight. The Company recorded deferred income tax assets related to these NOLs of $18.5 million and $25.8 million as of December 31, 2014 and December 31, 2013, respectively, and offsetting valuation allowances of $8.1 million and $25.8 million, respectively, 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 December 31, 2014 and December 31, 2013, the Company had non-U.S. NOL carryforwards of $90.6 million and $86.4 million, respectively, of which $62.4 million and $65.7 million, respectively, were generated by Knight in periods prior to the Mergers. The Company recorded a foreign deferred income tax asset of $19.5 million and $18.9 million for these NOL carryforwards as of December 31, 2014 and December 31, 2013, respectively, along with an offsetting U.S. federal deferred tax liability of $19.5 million and $18.9 million, respectively, 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. The Company had tax credit carryforwards comprising general business credit carryforwards of $4.4 million and $2.5 million, at December 31, 2014 and December 31, 2013, respectively, and alternative minimum tax credit carryforwards $6.8 million at each December 31, 2014 and December 31, 2013.
At December 31, 2014, and December 31, 2013, the Company had unrecognized tax benefits, respectively, of $2.2 million and $1.5 million, all of which would affect the Company's effective tax rate if recognized.
The following table reconciles the beginning and ending amount of unrecognized tax benefits (in thousands):


	For the year ended December 31,
	2014				2013
Balance at beginning of period	$	1,464			$	—

Increases based on tax positions related to prior periods	1,843				1,464

Decreases based on tax positions related to prior periods	(995		)		—

Decreases related to settlements with taxing authorities	—				—

Balance at the end of the period	$	2,312			$	1,464

As of December 31, 2014, the Company is subject to U.S. Federal income tax examinations for the tax years 2009 through 2013, and to non U.S. income tax examinations for the tax years 2007 through 2013. 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 does not anticipate that any adjustments would result in a material change to its results of operations or financial condition.
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	12 Months Ended
	Dec. 31, 2014
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 years ended December 31, 2014, 2013 and 2012 (in thousands):


		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments		Total
	Balance January 1, 2012	$	—		$	—	$	—

	Other comprehensive income	114,319			—		114,319

	Balance December 31, 2012	114,319			—		114,319

	Other comprehensive income	13,689			1,365		15,054

	Amount reclassified from Accumulated Other Comprehensive Income (1)	(127,972		)	—		(127,972		)

	Net current-period other comprehensive (loss) income	(114,283		)	1,365		(112,918		)

	Balance December 31, 2013	36			1,365		1,401

	Other comprehensive income	316			416		732

	Balance December 31, 2014	$	352		$	1,781	$	2,133

	(1) As a result of the Mergers, the Company recorded a non-cash gain of $128.0 million on its investment in Knight Common Stock, which it recognized within Investment income and other, net on the Consolidated Statements of Operations for the year ended December 31, 2013 and reversed any previous unrealized gains out of Other comprehensive income.

Writedowns_and_Other_Charges

Writedowns and Other Charges	12 Months Ended
Writedowns and Other Charges	Dec. 31, 2014
Restructuring and Related Activities [Abstract]
Writedowns and Other Charges	Writedowns and Other Charges
	Writedown of capitalized debt costs
	In 2014 and 2013, the Company made principal repayments under the Credit Agreement of $235.0 million and $300.0 million, respectively. As a result, for the years ended 2014 and 2013, $9.6 million and $13.2 million, respectively, were written down.
	Lease loss accrual
	For the years ended 2014 and 2013, the Company recorded $8.6 million and $7.1 million, respectively, of net lease loss accruals related to excess real estate capacity.
	Writedown of assets
	For the year ended 2013, the Company recorded $7.7 million in writedown of assets primarily related to leasehold improvements and fixed assets.

Earnings_Per_Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2014
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 KCG 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 were exercised and restricted awards were to vest.
	The number of such RSUs, options, warrants and SARs excluded from the EPS calculation was approximately 28.3 million and 29.1 million for the years ended December 31, 2014 and 2013, 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 KCG 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 year ended December 31, 2014, 2013 and 2012 (in thousands):


		For the year ended December 31,
		2014				2013					2012
		Numerator /		Denominator /		Numerator /			Denominator /		Numerator /		Denominator /
		net income		shares		net income			shares		net income		shares
	Income from continuing operations and shares used in basic calculations	$	62,434	112,854		$	126,092		80,143		16,151		48,970

	Effect of dilutive stock based awards
	Restricted awards			3,579					855				—

	Stock options and SARs			101					17				—

	Income from continuing operations and shares used in diluted calculations	$	62,434	116,534		$	126,092		81,015		$	16,151	48,970

	Loss from continuing operations allocated to preferred and participating units	$	—			$	(21,565	)			$	1,092

	Income from continuing operations attributable to common shareholders	$	62,434			$	147,657				$	15,059

	Basic earnings per common share from continuing operations			$	0.55				$	1.84			$	0.31

	Diluted earnings per common share from continuing operations			$	0.54				$	1.82			$	0.31

	Prior to the Mergers, GETCO units comprised preferred and common units, and net income was allocated among the various classes of units based upon participation rights in undistributed earnings. The number of shares used to calculate EPS for 2013 are GETCO units converted into KCG shares using an exchange ratio as detailed in the Merger Agreement.

Significant_Clients

Significant Clients	12 Months Ended
Significant Clients	Dec. 31, 2014
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 years ended December 31, 2014, 2013 or 2012.

Commitments_and_Contingent_Lia

Commitments and Contingent Liabilities	12 Months Ended
	Dec. 31, 2014
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. Knight has since been named as a defendant in two putative class action complaints (one of which was voluntarily dismissed) and one derivative lawsuit, all of which relate to the technology issue. Knight has 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 "August 6, 2012 recapitalization").
	After the announcement on December 19, 2012 of the signing of the Merger Agreement, Knight, GETCO, GA-GTCO, as well as the individual members of the Knight's Board of Directors prior to the Mergers (the “Individual Defendants”), were named as defendants in several lawsuits brought by certain purported Knight stockholders challenging the proposed Mergers. The lawsuits generally alleged, among other things, that the Mergers failed to properly value Knight, that the Individual Defendants breached their fiduciary duties in approving the Merger Agreement and that those breaches were aided and abetted by GETCO and GA-GTCO. The lawsuits, among other things, sought to enjoin the defendants from completing the Mergers on the agreed-upon terms, rescission of the Mergers (to the extent the Mergers have already been consummated), monetary relief and attorneys' fees and costs.
	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. 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 paid an inflated price. The defendants filed a motion to dismiss the second amended complaint on February 18, 2014. The motion was fully briefed as of June 5, 2014, and is before the court for decision.
	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 litigation. 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. The District Court has set March 16, 2015 for a preliminary approval hearing. Under the terms of the proposed settlement, Knight has agreed that certain of its insurance carriers will pay $13 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. Because the full amount of the proposed settlement will be paid by the Company’s insurance carriers, the settlement will not have an effect on the Company’s results of operations. Though the Company believes the likelihood of approval of the settlement is probable, we cannot predict with certainty the outcome of the litigation, and if the settlement is not finally approved by the Court, we believe that we have meritorious defenses to the claims in the operative complaint.
	As noted above, Knight received several demand letters requesting that it commence a lawsuit against certain directors and officers for alleged breaches of fiduciary duties, waste, wrongdoing, mismanagement and/or demanding that it produce certain books and records pursuant to Delaware law concerning the technology issue and the August 6, 2012 recapitalization.
	Mergers Litigation
	Delaware Litigation. On December 28, 2012, a purported stockholder class action complaint was filed in the Court of Chancery of the State of Delaware, captioned Ann Jimenez McMillan v. Thomas M. Joyce, et al., Case No. 8163-VCP. The complaint names as defendants Knight, the Individual Defendants, GETCO, and GA-GTCO, LLC. The complaint generally alleges, among other things, that the Individual Defendants violated their fiduciary duties by accepting an inadequate merger price, approving the transaction despite material conflicts of interest, and agreeing to a number of improper deal protection devices and voting agreements, which allegedly make it less likely that other bidders would make successful competing offers for Knight. The complaint also alleges that Knight, GETCO, and GA-GTCO, LLC aided and abetted these purported breaches of fiduciary duties. The relief sought includes, among other things, an injunction prohibiting consummation of the Mergers, rescission of the Mergers (to the extent the Mergers have already been consummated), and attorneys' fees and costs. On December 28, 2012, a purported stockholder class action complaint was filed in the Court of Chancery of the State of Delaware, captioned Chrislaine Dominique v. Thomas M. Joyce, et al., Case No. 8159-VCP. The complaint names as defendants Knight, the Individual Defendants, GETCO, and GA-GTCO, LLC. The complaint generally alleges, among other things, that the Individual Defendants violated their fiduciary duties by accepting an inadequate merger price, approving the transaction despite material conflicts of interest, including that they were appointed by an investor group that included GETCO, and agreeing to a number of improper deal protection devices, which allegedly make it less likely that other bidders would make successful competing offers for Knight. The complaint also alleges that Knight and GETCO aided and abetted these purported breaches of fiduciary duties. The relief sought includes, among other things, an injunction prohibiting consummation of the Mergers, rescission of the Mergers (to the extent the Mergers have already been consummated), and attorneys' fees and costs. On January 31, 2013, the Court of Chancery consolidated for all purposes the McMillan and Dominique actions into a single action captioned In re Knight Capital Group, Inc. Shareholder Litigation, C.A. No. 8159-VCP. On March 5, 2013, the co-lead plaintiffs in the Delaware Consolidated Action filed an amended complaint and motions for expedited discovery and a preliminary injunction. In addition to the allegations in the initial complaints, the Delaware amended complaint contains allegations that the Knight Board of Directors breached its fiduciary duties by providing stockholders with allegedly deficient disclosures about the proposed transaction in the Company's Preliminary Form S-4, filed with the SEC on February 13, 2013 (the “Preliminary Proxy”).
	New Jersey Litigation. On December 31, 2012, a purported stockholder class action complaint was filed in the Superior Court of New Jersey, Chancery Division of Hudson County, NJ, captioned Charles Bryan v. Knight Capital, et al., Case No. HUD-C-001-13. The complaint names as defendants Knight, the Individual Defendants, Jefferies & Company, Inc., Jefferies High Yield Trading, LLC, TD Ameritrade Holding Corp., Blackstone Capital Partners VI L.P., Blackstone Family Investment Partnership VI-ESC L.P., Blackstone Family Investment Partnership VI L.P., Stephens Investments Holdings LLC, Stifel Financial Corp., GETCO Strategic Investments, LLC, GETCO Holding Company LLC, and GA-GTCO, LLC. The complaint generally alleges that the Individual Defendants breached their fiduciary duties by accepting an inadequate merger price, agreeing to a number of improper deal protection devices and voting agreements, which allegedly make it less likely that other bidders would make successful competing offers for Knight and approving the transaction despite material conflicts of interest, including that they were appointed by an investor group that included GETCO. The complaint further alleges that the entity defendants (except for Knight and GA-GTCO, LLC) breached alleged fiduciary duties in connection with the Individual Defendants' approval of the Mergers. The complaint also alleges that GETCO and GA-GTCO, LLC aided and abetted the Individual Defendants' purported breaches of fiduciary duty. The relief sought includes, among other things, an injunction prohibiting the consummation of the Mergers, rescission of the Mergers (to the extent the Mergers have already been consummated), and attorneys' fees and costs.
	On December 31, 2012, a purported stockholder class action complaint was filed in the Superior Court of New Jersey, Chancery Division of Hudson County, NJ, captioned James Ward v. Knight Capital, et al., Case No. HUD-C-0003-13. The complaint names as defendants Knight, the Individual Defendants, Jefferies & Company, Inc., Jefferies High Yield Trading, LLC, TD Ameritrade Holding Corp., Blackstone Capital Partners VI L.P., Blackstone Family Investment Partnership VI-ESC L.P., Blackstone Family Investment Partnership VI L.P., Stephens Investments Holdings LLC, Stifel Financial Corp., GETCO Strategic Investments, LLC, GETCO Holding Company LLC, and GA-GTCO, LLC. The complaint generally alleges that the Individual Defendants breached their fiduciary duties by accepting an inadequate merger price, agreeing to a number of improper deal protection devices and voting agreements, which allegedly make it less likely that other bidders would make successful competing offers for Knight and approving the transaction despite material conflicts of interest, including that they were appointed by an investor group that included GETCO. The complaint further alleges that the entity defendants (except for Knight and GA-GTCO, LLC) breached alleged fiduciary duties in connection with the Individual Defendants' approval of the Mergers. The complaint also alleges that GETCO and GA-GTCO, LLC aided and abetted the Individual Defendants' purported breaches of fiduciary duty. The relief sought includes, among other things, an injunction prohibiting the consummation of the Mergers, rescission of the Mergers (to the extent the Mergers have already been consummated), and attorneys' fees and costs. On February 20, 2013, Knight moved to dismiss or, in the alternative, stay the New Jersey actions in deference to the first-filed Delaware actions. The New Jersey court granted the motion on March 28, 2013, and ordered that the New Jersey actions be stayed for all purposes in deference to the first-filed Delaware actions.
	New York Litigation. On January 15, 2013, Knight, the Individual Defendants, GETCO, GA-GTCO, LLC and General Atlantic were named as defendants in an action entitled Joel Rosenfeld v. Thomas M. Joyce, et al., Case No. 6540147/2013, in the Supreme Court of the State of New York (New York County). The plaintiff, Joel Rosenfeld, is one of the stockholders mentioned above who previously sent Knight a derivative demand letter. Generally, this complaint asserts both derivative and class action claims. First, it purports to assert derivative claims, which allege, among other things, that the seven Knight directors who were serving as of August 1, 2012 breached their fiduciary duties and wasted corporate assets by failing to erect and oversee effective safeguards to prevent against technology issues, such as the one that occurred on August 1, 2012, for which Knight incurred a realized pre-tax loss of approximately $457.6 million. Second, it asserts putative class action claims resulting from the proposed Mergers for (1) breach of fiduciary duty against the Individual Defendants; and (2) aiding and abetting the purported breach of fiduciary duty against GETCO, GA-GTCO, LLC, and General Atlantic. The complaint generally alleges that the Individual Defendants breached their fiduciary duties by approving the Mergers at an inadequate price, agreeing to a number of improper deal protection devices and voting agreements, which allegedly make it less likely that other bidders would make successful competing offers for Knight, and that certain of Knight's directors have conflicts of interest in connection with the transaction, including that certain directors sought to enter into the transaction to avoid potential liability relating to the derivative claims asserted in the complaint. With respect to the merger claims, the plaintiff seeks, among other things, to enjoin the proposed Mergers, rescission of the proposed Mergers (to the extent they have already been consummated) and attorneys' fees. With respect to the derivative claims, the plaintiff seeks, among other things, an order requiring the Knight directors who were serving as of August 1, 2012 to pay restitution and/or compensatory damages in favor of Knight and/or the proposed class of Knight stockholders. On March 14, 2013, the plaintiff filed an amended complaint, which, in addition to the allegations in the initial complaint, contains allegations that the Knight Board of Directors breached its fiduciary duties by providing stockholders with allegedly deficient disclosures about the proposed transaction in the Preliminary Proxy. On March 21, 2013, the plaintiff moved by order to show cause for expedited discovery in support of his claims. The New York court issued an order on March 25, 2013, setting a hearing on the plaintiff's motion for April 4, 2013. On March 28, 2013, the parties in the New York action reached an agreement with respect to the matters raised in the plaintiff's motion and other aspects of the action, and as a result, on March 29, 2013, the plaintiff withdrew his motion for expedited discovery. On April 9, 2013, the New York court granted permission for the plaintiff to withdraw his motion.
	On June 10, 2013, the defendants entered into a memorandum of understanding with the plaintiffs in the Delaware shareholder actions and New York shareholder action regarding the settlement of those actions. In connection with the settlement, Knight and GETCO agreed to make supplemental disclosures to the joint proxy statement/prospectus filed with the SEC on May 28, 2013 (the “Proxy Statement”). In addition, Knight and GETCO agreed to make certain revisions to Knight's risk committee charter, as well as to KCG's risk committee charter.
	The memorandum of understanding contemplated that the parties would enter into a stipulation of settlement, which would be subject to customary conditions, including court approval following notice to Knight's former stockholders. It also contemplated that in the event that the parties enter into a stipulation of settlement, a hearing would be scheduled at which the Delaware Court of Chancery would consider the fairness, reasonableness and adequacy of the settlement. If the settlement is finally approved by the court, it would resolve and release all claims that were brought or could have been brought in the Delaware, New York, and New Jersey shareholder actions, including claims challenging any aspect of the Mergers, the Merger Agreement, or any disclosure made in connection therewith, pursuant to terms that will be disclosed to Knight's former stockholders prior to final approval of the settlement. In addition, in connection with the settlement, the parties contemplated that plaintiffs' counsel will file a petition in the Delaware Court of Chancery for an award of attorneys' fees and expenses to be paid by KCG.
	On June 5, 2014, the parties entered into a Stipulation and Agreement of Compromise, Settlement and Release (the “Stipulation”). As contemplated in the memorandum of understanding, the Stipulation provides, among other things, that in exchange for the supplemental disclosures and changes to Knight’s and KCG’s risk committee charters discussed above, and upon final approval by the Delaware Court of Chancery, the Delaware and New York stockholder plaintiffs, and a class of former Knight stockholders that includes such plaintiffs, will finally and fully resolve and release all claims that were brought or could have been brought in the Delaware, New York, and New Jersey shareholder actions, including claims challenging any aspect of the Mergers, the Merger Agreement, or any disclosure made in connection therewith. The Stipulation further provides that counsel for the Delaware and New York shareholder plaintiffs will seek, and the defendants will not oppose, court approval of an award of attorneys’ fees and costs in an amount not to exceed $490,000. The Stipulation and the settlement it contemplates, is not, and should not be construed as, an admission of wrongdoing or liability by any of the defendants. Nonetheless, the defendants entered into the settlement to avoid the risk of the stockholder actions delaying or adversely affecting the Mergers, to minimize the substantial expense, burden, distraction and inconvenience of continued litigation, and to fully and finally resolve the claims in the stockholder actions.
	On June 9, 2014, the parties to the Stipulation filed the Stipulation and associated exhibits with the Delaware Court of Chancery, seeking among other things, preliminary approval of the settlement, conditional certification of a non-opt-out class of former Knight stockholders for settlement purposes only, and the scheduling of a final settlement hearing.
	On June 17, 2014, the Delaware Court of Chancery entered an order preliminarily approving the settlement as set forth in the Stipulation, conditionally certifying a non-opt-out class of former Knight stockholders for settlement purposes only pursuant to Court of Chancery Rules 23(a), 23(b)(1), and 23(b)(2), and scheduling a final settlement hearing to be held on September 26, 2014. The Court’s preliminary order provided that at the final settlement hearing, the Court would, among other things, determine whether to finally certify the non-opt-out class of former Knight stockholders, determine whether the settlement is fair, reasonable, and adequate to the class and should be approved by the Court, determine whether to enter a final order and judgment dismissing the action with prejudice, consider the application for attorneys’ fees and costs by counsel for the Delaware and New York shareholder plaintiffs, and rule on such other matters as the Court may deem appropriate. The Court’s preliminary order further provided that members of the conditionally-certified class would receive notice of the settlement hearing at least 45 days prior to the hearing, which notice would describe, among other things, the material terms of the settlement and the procedures for class members to follow if they wished to object to the settlement and/or be heard at the final hearing.
	On September 26, 2014, the Delaware Court of Chancery held a settlement hearing to determine, among other things, whether the proposed settlement is fair, reasonable, and adequate to the class. Following the hearing, the Delaware Court issued a final order and judgment that, among other things, (a) determined that the defendants had complied with the notice requirements under Delaware Court of Chancery Rule 23; (b) certified a non-opt-out class action pursuant to Delaware Court of Chancery Rules 23(a) and 23(b)(1) and (b)(2); (c) found the proposed settlement to be fair, reasonable and adequate and in the best interests of the class, and approved it pursuant to Delaware Court of Chancery Rule 23(e); (d) released the defendants from any claims that were brought, or could have been brought, by the plaintiffs in the Delaware or New York shareholder actions as well as from claims that could have been brought by the shareholders who sent demand letters to the Company described in the previous section; and (e) granted an award of attorneys’ fees, costs and expenses to attorneys for plaintiffs in the Delaware and New York shareholder actions in the amount of $425,000. Pursuant to the order and final judgment, the Delaware shareholder action was dismissed with prejudice.
	As contemplated by the settlement, on October 9, 2014, the plaintiff in the New York shareholder action filed a Stipulation of Discontinuance with Prejudice. In addition, on September 2, 2014, the plaintiffs in the New Jersey shareholder actions filed a Notice of Voluntary Dismissal with Prejudice in the New Jersey Court.
	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 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 and SROs that arise from its business activities. 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 regulators, the New York Attorney General, Congress 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 December 31, 2014, the obligations have a weighted-average interest rate of 3.52% 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 December 31, 2014 consist of (in thousands):


		Minimum Payments
	2015	$	4,223

	2016	2,126

	2017	620

	Total	$	6,969

	The total interest expense related to capital leases for the years ended December 31, 2014, and 2013 included in the Consolidated Statements Operations is as follows (in thousands):


		For the year ended December 31,
		2014			2013			2012
	Interest expense - Capital leases	$	370		$	700		$	1,376

	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 $19.7 million and $16.0 million for the years ended December 31, 2014 and 2013, 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 December 31, 2014, 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
	Year ending December 31, 2015	$	28,224		$	4,056		$	24,168

	Year ending December 31, 2016	27,762			3,488			24,274

	Year ending December 31, 2017	26,211			2,983			23,228

	Year ending December 31, 2018	25,354			2,667			22,687

	Year ending December 31, 2019	23,316			2,389			20,927

Thereafter through December 31, 2027	64,441			10,244			54,197

Total	$	195,308		$	25,827		$	169,481

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 December 31, 2014, 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 joint venture. 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. At December 31, 2014, the Company had $9.2 million in compensation guarantees payable through December 31, 2015. There were no compensation guarantees at December 31, 2013.

Net_Capital_Requirements

Net Capital Requirements

12 Months Ended

Dec. 31, 2014

Brokers and Dealers [Abstract]

Net Capital Requirements

Net capital requirements

The Company's U.S. registered broker dealer is subject to the SEC’s Uniform Net Capital Rule, which requires the maintenance of minimum net capital. In 2014, the Company's U.S. registered broker dealer was in compliance with its capital adequacy requirements.

The following table sets forth the net capital levels and requirements for the Company’s U.S. registered broker dealer subsidiary at December 31, 2014 as filed in its regulatory filings (in thousands):

Entity

Net Capital

Excess Net

Requirement

Capital

KCG Americas LLC

285,231

1,000

284,231

The Company's U.K. registered broker dealers are subject to certain financial resource requirements of Financial Conduct Authority ("FCA") while the Company's Singapore and Australian broker dealers are subject to certain financial resource requirements of the Securities and Futures Commission and the Australian Securities and Investment Commission, respectively. The following table sets forth the financial resource requirement for the following foreign regulated broker dealers at December 31, 2014 (in thousands):

Entity

Financial

Resource

Excess

Resources

Requirement

Financial

Resources

KCG Europe Limited

187,940

124,577

63,363

GETCO Europe Limited

7,254

1,759

5,495

The businesses of GETCO Europe Limited were combined into KCG Europe Limited in the second quarter of 2014 but the legal entity itself has not officially dissolved. The consolidation of the Company's U.K. broker dealers allowed for the release of approximately $45.0 million in excess capital during year ended December 31, 2014. The Company is in the process of withdrawing GETCO Europe Limited's membership with the FCA.

Financial_instruments_with_Off

Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk	12 Months Ended
	Dec. 31, 2014
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 December 31, 2014 and 2013 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-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 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		15,245			8,894			14,847			9,105

	Total			$	169,423		458,242			$	139,811		469,292



				31-Dec-13
		Financial Statements		Assets						Liabilities
		Location		Fair Value			Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	89		892			$	142		533

	Forward contracts	Other assets		6,913			1			6,501			1

	Forward contracts(1)	Financial instruments sold, not yet purchased, at fair value		—			—			5,014			1

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		223			1,069			1,089			1,046

	Swap contracts	Receivable from/Payable to brokers, dealers and clearing organizations		—			—			18			1

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		339,798			730,020			252,282			755,947

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		4,815			18,280			2,259			15,202

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		3,392			10,629			1,773			3,806

	Total			$	355,230		760,891			$	269,078		776,537


	(1)	Designated as hedging instrument.


					Gain (Loss) Recognized
			Financial Statements		For the year ended December 31,
		Location		2014			2013				2012
Derivative instruments not designated as hedging instruments:
Foreign currency
Futures contracts		Trading revenues, net		$	10,535		$	12,191			$	19,699

Forward contracts		Investment Income and other, net		526			—				—

Equity
Futures contracts		Trading revenues, net		25,247			50,073				76,089

Swap contracts		Trading revenues, net		5,277			11,736				8,116

Listed options (1)		Trading revenues, net		(37,439		)	37,035				50,446

Fixed income
Futures contracts		Trading revenues, net		31,277			80,511				81,933

Commodity
Futures contracts		Trading revenues, net		55,295			62,215				35,952

				$	90,718		$	253,761			$	272,235

Derivative instruments designated as hedging instruments:
Foreign exchange - forward contract		Accumulated other comprehensive income		$	—		$	(3,298	)		$	—


(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.
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	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Business Segments	Business Segments
	As of December 31, 2014, 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, foreign exchange, 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) an institutional spot foreign exchange ECN; (iv) a fixed income ECN that also offers trading applications; and (v) 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 such as self-clearing services, including stock lending activities.
	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 year ended December 31, 2014:
	Revenues	$	901,152	$	345,710		$	69,369		$	1,316,232

	Pre-tax earnings	146,713		11,056			(72,582		)	85,187

	Total assets	4,401,021		786,734			1,642,899			6,830,654

	For the year ended December 31, 2013:
	Revenues	$	688,197	$	197,765		$	141,374		$	1,027,336

	Pre-tax earnings	103,612		(25,794		)	(52,842		)	24,978

	Total assets	3,939,965		1,106,448			1,950,591			6,997,004

	For the year ended December 31, 2012:
	Revenues	$	495,427	$	36,211		$	19,596		$	551,234

	Pre-tax earnings	34,887		(7,330		)	(1,130		)	26,427

	Total assets	1,408,516		33,623			245,397			1,687,536

	In the first quarter of 2014, the Company began to charge the Market Making and Global Execution Services segments for the cost of aggregate debt interest. The interest amount charged to each of the segments is determined based on capital limits and requirements. Historically, debt interest was included within the Corporate and Other segment. This change in the measurement of segment profitability has no impact on the consolidated results and will only be reported prospectively, and will not be reflected in financial results prior to January 1, 2014. For the year ended December 31, 2014 debt interest expense included in the results of the Market Making and Global Execution Services segments was $24.7 million and $7.1 million, respectively.
	Included in total assets within Global Execution Services at December 31, 2014 is $40.5 million related to KCG Hotspot. As noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", such assets are included as Assets held for sale at December 31, 2014. See Footnote 27 "Subsequent Event"
	The Company operates in the U.S. and internationally, primarily in Europe and Asia. The following table presents Revenues by geographic area.


		U.S.		International			Consolidated
							Total
	For the year ended December 31, 2014:
	Revenues	$	1,127,088	$	189,144		$	1,316,232

	For the year ended December 31, 2013:
	Revenues	$	834,410	$	192,926		$	1,027,336

	For the year ended December 31, 2012:
	Revenues	$	340,117	$	211,117		$	551,234

Condensed_Financial_Statements

Condensed Financial Statements of KCG Holdings, Inc. (parent only)	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Financial Statements of KCG Holdings, Inc. (parent only)	Condensed Financial Statements of KCG Holdings, Inc. (parent only)
	Presented below are the Condensed Statements of Financial Condition, Operations and Cash Flows for the Company on an unconsolidated basis.


	Statements of Financial Condition
	KCG Holdings, Inc. (parent only)


		December 31,
		2014			2013
		(in thousands)
	Assets
	Cash and cash equivalents	$	302,700		$	253,017

	Receivable from subsidiaries	21,851			60,460

	Investments in subsidiaries	1,122,254			1,359,779

	Income taxes receivable	13,999			90

	Deferred tax asset, net	137,009			143,028

	Subordinated loans to subsidiaries	280,000			250,000

	Other assets	19,397			21,182

	Total assets	$	1,897,210		$	2,087,556

	Liabilities and equity
	Liabilities
	Accrued compensation expense	$	13,208		$	—

	Payable to subsidiaries	42,478			25,705

	Accrued expenses and other liabilities	13,947			12,314

	Debt	305,000			540,000

	Total liabilities	374,633			578,019

	Total equity	1,522,577			1,509,537

	Total liabilities and equity	$	1,897,210		$	2,087,556

	Statements of Operations
	KCG Holdings, Inc. (parent only)


		For the year ended December 31,
		2014			2013			2012
		(in thousands)
	Revenues
	Investment income and other, net	$	3,415		$	3,717		$	5,981

	Total revenues	3,415			3,717			5,981

	Expenses
	Employee compensation and benefits	50,256			31,970			(12,251		)

	Interest	11,939			28,476			1,195

	Depreciation and amortization	—			698			1,395

	Professional fees	9,211			30,488			4,997

Business development	3,625			—			—

Other	25,460			29,829			1,493

Total expenses	100,491			121,461			(3,171		)

(Loss) income before income taxes and equity in earnings of subsidiaries	(97,076		)	(117,744		)	9,152

Income tax benefit	(35,972		)	(120,761		)	—

Income (loss) before equity in earnings of subsidiaries	(61,104		)	3,017			9,152

Equity in earnings of subsidiaries	122,206			123,155			6,999

Net income	$	61,102		$	126,172		$	16,151





Statements of Cash Flows
KCG Holdings, Inc. (parent only)


	For the year ended December 31,
	2014			2013			2012
	(In thousands)
Cash flows from operating activities
Net income	$	61,102		$	126,172		$	16,151

Adjustments to reconcile net income to net cash used in operating activities
Equity in earnings of subsidiaries	(122,206		)	(123,155		)	(6,999		)
Writedown and amortization of debt issuance costs	12,548			16,931			52

Amortization of intangibles	—			698			1,395

Stock-based compensation	16,997			—			—

Decrease (increase) in operating assets
Subordinated loan receivable	(30,000		)	(250,000		)	—

Receivable from subsidiaries	38,609			(67,523		)	(97,375		)

Deferred tax asset	(13,909		)	(144,109		)	—

Income taxes receivable	6,019			(90		)	—

Other assets	(10,763		)	(15,351		)	31,031

(Decrease) increase in operating liabilities
Payable to subsidiaries	16,773			(2,024		)	11,756

Accrued compensation expense	8,839			—			—

Accrued expenses and other liabilities	1,633			59,561			5,106

Net cash used in operating activities	(14,358		)	(398,890		)	(38,883		)
Cash flows from investing activities
Cash acquired upon acquisition of Knight Capital Group, Inc.	—			509,133			—

Cash received from sale of Urban Financial of America, LLC	—			85,406			—

Dividends received from subsidiaries	807,715			646,425			161,606

Capital contributions to subsidiaries	(397,089		)	(238,909		)	(70,794		)
Net cash provided by investing activities	410,626			1,002,055			90,812

Cash flows from financing activities
Proceeds from issuance of Credit Agreement	—			535,000			—

Partial repayment of Credit Agreement	(235,000		)	(300,000		)	—

Proceeds from issuance of Senior Secured Notes	—			305,000			—

Repayment of notes payable	—			(15,000		)	—

Payment of debt issuance costs	—			(34,592		)	—

Issuance of equity to General Atlantic	—			55,000			—

Payment to former Knight Capital Group, Inc. stockholders	—			(720,000		)	—

Repayment of Knight Convertible Notes	—			(257,741		)	—

Funding of collateral account for Knight Convertible Notes	—			(117,259		)	—

Payment out of collateral account for Knight Convertible Notes	—			117,259			—

Proceeds from stock issuance	—			—			12,320

Member distributions	—			(21,002		)	(62,234		)

Cost of common stock repurchased	(111,585		)	(11,324		)	—

Net cash used in financing activities	(346,585		)	(464,659		)	(49,914		)
Effect of exchange rate changes on cash and cash equivalents	—			—			—

Increase in cash and cash equivalents	49,683			138,506			2,015

Cash and cash equivalents at beginning of period	253,017			114,511			112,496

Cash and cash equivalents at end of period	$	302,700		$	253,017		$	114,511

Supplemental disclosure of cash flow information:

Cash paid for interest	$	28,426		$	26,239		$	1,116

Cash paid for income taxes	$	15,456		$	365		$	—

The condensed financial statements of KCG Holdings, Inc. (parent only; the “Parent Company”) should be read in conjunction with the Consolidated Financial Statements of the Company and the Notes thereto. As noted in Footnote 1 "Organization and Description of Business", the Mergers were treated as a purchase of Knight by GETCO for accounting and financial reporting purposes.
The Statements of Operations and Cash Flows included in this footnote for the year ended December 31, 2013 include the combined financial results and cash flows for GETCO Holding Company, LLC for the six month period January 1, 2013 to June 30, 2013 and for KCG Holdings, Inc. for the six month period July 1, 2013 to December 31, 2013. These combined Statements of Operations and Cash Flows were presented for comparability to the prior year, 2012.

Subsequent_Event_Subsequent_Ev

Subsequent Event Subsequent Event	12 Months Ended
Subsequent Event Subsequent Event	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event
	The Company has evaluated subsequent events through the date the Consolidated Financial Statements were issued. The Company did not note any subsequent events requiring adjustment or disclosure in the Consolidated Financial Statements except for the following.
	In January 2015, the Company announced the sale of KCG Hotspot to BATS. Under the terms of the agreement, the Company will receive $365.0 million in cash upon the close. In addition, the Company and BATS have agreed to share certain tax benefits, which could result in further payments to the Company of up to approximately $70.0 million in the three-year period following the close.
	The transaction, which is subject to customary closing conditions, is expected to close in March 2015. The Company does not believe that this sale represents a strategic shift that will have a major effect on the Company’s operations and financial results, and therefore KCG Hotspot does not meet the requirements to be treated as a discontinued operation.
	The sale of KCG Hotspot is structured as an asset sale for income tax purposes and BATS and KCG have agreed to share certain related tax benefits that potentially accrue to BATS after the closing of the transaction. KCG will share in 70% of the actual tax benefits to BATS for the first three years after the closing and 50% of the actual tax benefits thereafter (the “Annual Tax Benefits”). However, KCG has a one-time option exercisable within 30 days of the third anniversary of the closing of the transaction to terminate the continued tax sharing arrangement in exchange for a one-time payment of $50.0 million, which BATS has the right to exercise after KCG’s option expires. However, the receipt of the Annual Tax Benefits by KCG is subject to BATS having sufficient net income to receive the tax benefits. BATS’s net income could decrease due to numerous factors, which are outside of the control of KCG. In addition, any decrease in the corporate tax rates applicable to BATS could reduce the size or certainty of the Annual Tax Benefits.
	In addition, KCG will not be entitled to the Annual Tax Benefits in the first three years if, during the five month period beginning on January 27, 2015 (subject to extension in limited cases) (the “Measurement Period”), the average daily notional FX trading volume on KCG Hotspot for any 60 trading day rolling period is less than or equal to a threshold set at 70% of the average daily notional FX trading volume on KCG Hotspot for the month with the lowest trading volume over the last two calendar years (the “Trigger Event”) and, after the Trigger Event but during the Measurement Period, the average daily notional FX trading volume on Hotspot for any 15 trading day rolling period is not 80% or more of the average daily notional FX trading volume on Hotspot for the 2014 calendar year and no subsequent Trigger Event occurs during the Measurement Period.

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
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 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 11 "Investments", as a result of the merger of BATS Global Markets, Inc. ("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 securities 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	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.
	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 includes commission equivalents earned on institutional client orders and commissions on futures transactions, and related expenses are also recorded on a trade date basis. 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.
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 either 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.
	Prior to the Mergers, GETCO had a strategic investment in Knight which was classified as available for sale and held at fair value with any unrealized gains or losses recorded in Other comprehensive income or loss. As a result of the Mergers, the Company recognized a non-cash gain of $128.0 million on its investment in Knight Common Stock, which it recorded within Investment income and other, net on the Consolidated Statement of Operations for the year ended December 31, 2013 and reversed any previous unrealized gains out of Other comprehensive income.
	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 the existence of an impairment. 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 this 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 years ended December 31, 2014, 2013 and 2012, the Company recorded a losses of $2.0 million, $4.1 million and a gain of $0.2 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. For qualifying net investment hedges, any gains or losses, to the extent effective, are included in Accumulated other comprehensive income on the Consolidated Statements of Financial Condition. The ineffective portion, if any, is recorded in Investment income and other, net on the Consolidated Statements of Operations.
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
	Prior to the Mergers, GETCO and the majority of its subsidiaries were treated as partnerships or disregarded entities for U.S. income tax purposes and, accordingly, were not subject to federal income taxes. Instead, the former GETCO members were liable for federal income taxes on their proportionate share of taxable income. Upon completion of the Mergers, the Company became a corporation subject to U.S. corporate income taxes and, following the Mergers, 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.
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 March 2013, the Financial Accounting Standards Board (“FASB”) issued an Accounting Standard Update (“ASU”) concerning the parent's accounting for the cumulative translation adjustment upon derecognition of certain subsidiaries or groups of assets within a foreign entity or of an investment in a foreign entity. This ASU provides for the release of the cumulative translation adjustment into net income when a parent sells a part or all of its investment within a foreign entity, no longer holds a controlling interest in an investment in a foreign entity or obtains control of an investment in a foreign entity that was previously recognized as an equity method investment. This ASU was effective for reporting periods beginning after December 15, 2013. The adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.
	In July 2013, the FASB issued an ASU to clarify the financial statement presentation of an unrecognized tax benefit when a NOL carryforward, a similar tax loss, or a tax credit carryforward exists. This ASU requires entities to present an unrecognized tax benefit as a reduction of a deferred tax asset for a NOL carryforward whenever the NOL or tax credit carryforward would be available to reduce the additional taxable income or tax due if the tax position is disallowed. The ASU was effective for reporting periods beginning after December 15, 2013. The adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.
	In April 2014, the FASB issued an ASU that amends the requirements for reporting discontinued operations. Under the new guidance, discontinued operations reporting will be 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.
	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 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 guidance is effective for reporting periods beginning after December 15, 2014. Early adoption is not permitted. 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 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.

Merger_of_GETCO_and_Knight_Tab

Merger of GETCO and Knight (Tables)	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Schedule of Business Acquisitions, by Acquisition	Amounts allocated to intangible assets and goodwill, and the amortization period for intangible assets with finite useful lives, were as follows (dollars in thousands):


							Amortization
			Amount				Years
	Technology		$	110,504			5 years

	Customer relationships		35,000				9 - 11 years

	Trade names		4,000				10 years

	Trading rights (1)		5,921				7 years

	Intangible assets		155,425

	Goodwill		12,666

	Total		$	168,091


	-1	Trading rights include both assets with a finite useful life and assets with an indefinite useful life. The 7 years amortization period only applies to assets with a finite useful life.
	The following table reflects the allocation of the purchase price to the assets acquired and liabilities assumed at the acquisition date (in thousands):


	Identifiable Net Assets
	Cash and cash equivalents				$	509,133

	Cash and cash equivalents segregated under federal and other regulations				203,045

	Financial instruments owned				1,937,929

	Securities borrowed				1,158,981

	Receivable from brokers, dealers and clearing organizations				1,369,474

	Fixed assets and leasehold improvements				80,280

	Investments				106,353

	Intangible assets				155,425

	Assets within discontinued operations				5,607,063

	Deferred tax asset, net				65,465

	Other assets				140,933

	Total Assets				$	11,334,081


	Financial instruments sold, not yet purchased				$	1,512,983

	Collateralized financings				1,166,211

	Payable to brokers, dealers and clearing organizations				635,914

	Payable to customers				527,918

	Accrued compensation expense				107,409

	Accrued expenses and other liabilities				130,010

	Liabilities within discontinued operations				5,518,168

Debt				375,000

Total Liabilities				$	9,973,613


Total identified assets acquired, net of assumed liabilities				1,360,468


Goodwill				12,666


Total Purchase Price				$	1,373,134

Significant_Accounting_Policie2

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
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 year ended December 31,
		2014			2013			2012
	Interest Income	$	14,363		$	10,187		$	3,070

	Interest Expense	(13,742		)	(10,724		)	(5,427		)
	Interest, net	$	621		$	(537	)	$	(2,357	)

Net Trading Revenue Including Dividend Income and Expense	Trading revenues, net includes dividend income and expense as follows (in thousands):


		For the year ended December 31,
		2014			2013			2012
	Dividend Income	$	45,910		$	26,237		$	2,253

	Dividend Expense	$	(38,444	)	$	(21,630	)	$	(1,599	)

Discontinued_Operations_Assets1

Discontinued Operations & Assets and LIabilities Held for Sale(Tables)	12 Months Ended
	Dec. 31, 2014
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 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

	In January 2015, KCG entered into an agreement to sell KCG Hotspot to BATS. See Footnote 27 “Subsequent Event” for additional information.
	The revenues and results of operations of discontinued operations are summarized as follows (in thousands):


		For the year ended December 31, 2014			For the six months ended December 31, 2013
	Revenues & Gain on sale	$	(1,148	)	$	39,868


	Expenses:
	Compensation	$	70		$	14,068

	Payments for order flow	—			9,885

	Execution and clearance fees	—			5,038

	Other expenses	930			10,748

	Total Expenses	1,000			39,739

	Pre-tax (loss) income from discontinued operations	(2,148		)	129

	Income tax benefit (expense)	816			(49		)

	Loss (Income) from discontinued operations, net of tax	$	(1,332	)	$	80

Assets_Segregated_or_Held_in_S1

Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations (Tables)	12 Months Ended
	Dec. 31, 2014
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 ("Segregated assets") primarily relate to the Company’s regulated businesses and consist of the following (in thousands):


		December 31,		31-Dec-13
		2014
	Cash and cash equivalents segregated under federal or other regulations	$	3,361	$	183,082

	Receivable from brokers, dealers and clearing organizations(1)	—		304,294

	Total assets segregated or held in separate accounts under federal or other regulations	$	3,361	$	487,376

	(1) Segregated assets included within Receivable from brokers, dealers and clearing organizations comprise cash and cash equivalents and U.S. government obligations primarily held as deposits with exchange clearing organizations.

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
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
	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

	Deferred compensation investments (3)	—			868			—			868

	Investment in Deephaven Funds (3)	—			146			—			146

	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, Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition.



		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	December 31, 2013	Level 1			Level 2			Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,298,785		$	—		$	—		$	2,298,785

	Listed options	339,798			—			—			339,798

	U.S. government and Non-U.S. government obligations	40,053			—			—			40,053

	Corporate debt	43,203			—			—			43,203

	Total Financial instruments owned, at fair value	2,721,839			—			—			2,721,839

Securities on deposit with clearing organizations (2)	170,235			—			—			170,235

Investment in CME Group (3)	3,925			—			—			3,925

Deferred compensation investments (3)	—			117			—			117

Investment in Deephaven Funds (3)	—			1,958			—			1,958

Total assets held at fair value	$	2,895,999		$	2,075		$	—		$	2,898,074

Liabilities
Financial instruments sold, not yet purchased, at fair value:
Equities (1)	$	1,851,006		$	—		$	—		$	1,851,006

Listed options	252,282			—			—			252,282

U.S. government obligations	15,076			—			—			15,076

Corporate debt	42,122			—			—			42,122

Foreign currency forward contracts	—			5,014			—			5,014

Total liabilities held at fair value	$	2,160,486		$	5,014		$	—		$	2,165,500

(1) Equities of $697.9 million have been netted by their respective long and short positions by CUSIP number.

(2)	Securities on deposit with clearing organizations consist of U.S. government obligations and are recorded within Receivable from brokers, dealers and clearing organizations on the Consolidated Statements of Financial Condition.
(3) Investment in CME Group, Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition.

Derivative_Financial_Instrumen1

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
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 December 31, 2014 and 2013 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-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 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		15,245				8,894			14,847				9,105

	Total			$	169,423			458,242			$	139,811			469,292



				31-Dec-13
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value				Contracts
	Foreign currency
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		$	89			892			$	142			533

	Forward contracts	Other assets		6,913				1			6,501				1

	Forward contracts(1)	Financial instruments sold, not yet purchased, at fair value		—				—			5,014				1

	Equity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		223				1,069			1,089				1,046

	Swap contracts	Receivable from/Payable to brokers, dealers and clearing organizations		—				—			18				1

	Listed options	Financial instruments owned/sold, not yet purchased, at fair value		339,798				730,020			252,282				755,947

	Fixed income
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		4,815				18,280			2,259				15,202

	Commodity
	Futures contracts	Receivable from/Payable to brokers, dealers and clearing organizations		3,392				10,629			1,773				3,806

	Total			$	355,230			760,891			$	269,078			776,537


	(1)	Designated as hedging instrument.
Fair Value of Derivative Instruments Gain Loss Recognized

					Gain (Loss) Recognized
			Financial Statements		For the year ended December 31,
			Location		2014				2013				2012
	Derivative instruments not designated as hedging instruments:
	Foreign currency
	Futures contracts		Trading revenues, net		$	10,535			$	12,191			$	19,699

	Forward contracts		Investment Income and other, net		526				—				—

	Equity
	Futures contracts		Trading revenues, net		25,247				50,073				76,089

	Swap contracts		Trading revenues, net		5,277				11,736				8,116

	Listed options (1)		Trading revenues, net		(37,439		)		37,035				50,446

	Fixed income
	Futures contracts		Trading revenues, net		31,277				80,511				81,933

	Commodity
	Futures contracts		Trading revenues, net		55,295				62,215				35,952

					$	90,718			$	253,761			$	272,235

	Derivative instruments designated as hedging instruments:
	Foreign exchange - forward contract		Accumulated other comprehensive income		$	—			$	(3,298	)		$	—


	(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.

Collateralized_Transactions_an1

Collateralized Transactions and Assets and Liabilities Subject to Netting (Tables)	12 Months Ended
	Dec. 31, 2014
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):


		December 31,			December 31,
		2014			2013
	Collateral permitted to be delivered or repledged	$	1,586,700		$	1,315,803

	Collateral that was delivered or repledged	1,485,267			1,231,468

	Collateral permitted to be further repledged by the receiving counterparty	147,696			142,938

Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge	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):


		December 31,			December 31,
		2014			2013
	Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge	$	536,124		$	552,242

	Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	979,652			676,956

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):


		December 31, 2014	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

		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

		Foreign currency forward contracts	60			—			60			—			—			60

		Swaps	85			—			85			—			—			85

		Futures	24,680			24,436			244			—			—			244

		Total Assets	$	1,823,018		$	24,436		$	1,798,582		$	1,591,619		$	15,782		$	191,181

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

		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

		Futures	24,436			24,436			—			—			—			—

		Total Liabilities	$	1,781,118		$	24,436		$	1,756,682		$	1,615,949		$	33,141		$	107,592

	(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 avalable 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 reverse repurchase agreements at broker dealer.


		31-Dec-13	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	$	339,798		$	—		$	339,798		$	—		$	—		$	339,798

		Securities borrowed	1,357,387			—			1,357,387			1,316,110			10,110			31,167

		Receivable from brokers, dealers and clearing organizations (3)	24,366			—			24,366			24,249			—			117

		Foreign currency forward contracts	6,913			6,501			412			—			—			412

		Futures	8,519			4,369			4,150			—			—			4,150

		Total Assets	$	1,736,983		$	10,870		$	1,726,113		$	1,340,359		$	10,110		$	375,644

		Liabilities
		Listed options	$	252,282		$	—		$	252,282		$	—		$	10,924		$	241,358

		Securities loaned	733,230			—			733,230			716,838			10,110			6,282

		Financial instruments sold under agreements to repurchase	640,950			—			640,950			640,948			—			2

		Foreign currency forward contracts	11,515			6,501			5,014			—			—			5,014

		Futures	5,263			5,263			—			—			—			—

	Swaps	18			—			18			—			—			18

	Total Liabilities	$	1,643,258		$	11,764		$	1,631,494		$	1,357,786		$	21,034		$	252,674

(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 avalable 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 reverse repurchase agreements at broker dealer.

Receivable_from_and_Payable_to1

Receivable from and Payable to Brokers, Dealers and Clearing Organizations (Tables)	12 Months Ended
	Dec. 31, 2014
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):


		December 31,		December 31, 2013
		2014
	Receivable:
	Clearing organizations and other	$	1,095,025	$	750,440

	Assets segregated or held in separate accounts under federal or other regulations (1)	—		304,294

	Securities failed to deliver	93,808		202,517

	Total Receivable	$	1,188,833	$	1,257,251

	Payable:
	Clearing organizations and other	$	350,627	$	425,196

	Securities failed to receive	325,462		48,912

	Total Payable	$	676,089	$	474,108

	(1) Segregated assets primarily related to the FCM Business, which was sold in the fourth quarter of 2014, included within Receivable from brokers, dealers and clearing organizations comprise cash and cash equivalents and U.S. government obligations primarily held as deposits with exchange clearing organizations.

Fixed_Assets_and_Leasehold_Imp1

Fixed Assets and Leasehold Improvements (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Schedule of Fixed Assets and Leasehold Improvements	Fixed assets and leasehold improvements comprise the following (in thousands):


		Depreciation	31-Dec-14			31-Dec-13
		Period
	Computer hardware and software	3 years	$	242,498		$	223,141

	Leasehold improvements	*	114,851			115,046

	Telephone systems and equipment	5 years	3,962			3,848

	Furniture and fixtures	7 years	12,120			12,227

			373,431			354,262

	Less - Accumulated depreciation and amortization		(239,380		)	(207,594		)
			$	134,051		$	146,668

	*Shorter of life of lease or useful life of assets

Investments_Tables

Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Summary of Investments	Investments comprise strategic investments, deferred compensation investments, and investment in the Deephaven Funds. Investments consist of the following (in thousands):


		December 31,		December 31,
		2014		2013
	Strategic investments:
	Investments accounted for under the equity method	$	86,328	$	110,460

	Investments held at fair value	4,435		3,925

	Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	8,949		8,953

	Total Strategic investments	99,712		123,338

	Deferred compensation investments	868		117

	Investment in Deephaven Funds	146		1,958

	Total Investments	$	100,726	$	125,413

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Goodwill	The following table summarizes the Company’s goodwill by segment (in thousands):


		December 31,				December 31, 2013
		2014
	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):


		December 31,				31-Dec-13
		2014
	Market Making
	Technology	$	50,542			$	53,315

	Trading rights	44,358				48,920

	Total	94,900				102,235

	Global Execution Services (1)
	Technology	18,200				38,682

	Customer relationships	10,833				33,278

	Trade names	850				3,800

	Total	29,883				75,760

	Corporate and Other
	Technology	10,500				13,500

	Total	$	135,283			$	191,495


	(1)	Excluded from the December 31, 2014 balance is $34.7 million of intangibles related to the KCG Hotspot which is held for sale. As noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", such amount is included in Assets of business held for sale at December 31, 2014.


			December 31,				31-Dec-13
			2014
	Technology (1)	Gross carrying amount	$	115,804			$	120,346

		Accumulated amortization	(36,562		)		(14,849		)
		Net carrying amount	79,242				105,497

	Trading rights (2)	Gross carrying amount	62,468				62,450

		Accumulated amortization	(18,110		)		(13,530		)
		Net carrying amount	44,358				48,920

	Customer relationships (3)	Gross carrying amount	13,000				35,000

		Accumulated amortization	(2,167		)		(1,722		)
		Net carrying amount	10,833				33,278

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

		Accumulated amortization	(150		)		(200		)
		Net carrying amount	850				3,800

	Total	Gross carrying amount	192,272				221,796

		Accumulated amortization	(56,989		)		(30,301		)
		Net carrying amount	$	135,283			$	191,495


	(1)	The weighted average remaining life for technology, including capitalized software, was approximately 3 and 4 years as of December 31, 2014 and December 31, 2013, respectively. Excluded from the December 31, 2014 balance is $13.1 million of technology assets related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", 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 and 8 years as of December 31, 2014 and December 31, 2013, respectively. As of December 31, 2014 and December 31, 2013, $6.8 million and $7.6 million, respectively, of trading rights had indefinite useful lives.

	(3)	Customer relationships relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 8 and 10 years as of December 31, 2014 and December 31, 2013, 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 the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", is included in Assets of business held for sale at December 31, 2014.

(4)	Trade names relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 9 and 10 years as of December 31, 2014 and December 31, 2013, respectively. Excluded from the December 31, 2014 balance is $2.6 million of trade name related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", 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 year ended December 31,
		2014				2013				2012
	Amortization expense	$	35,592			$	19,211			$	5,518

Summary of Estimated Amortization Expense for Future Years	As of December 31, 2014, the following table summarizes the Company’s estimated amortization expense for future periods (in thousands):


		Amortization
		expense
	For the year ended December 31, 2015	$	31,180

	For the year ended December 31, 2016	30,049

	For the year ended December 31, 2017	27,244

	For the year ended December 31, 2018	15,611

	For the year ended December 31, 2019	6,211

Debt_Tables

Debt (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Summary of Long-term Debt	The carrying value and fair value of the Company's debt is as follows (in thousands):


		December 31, 2014					December 31, 2013
		Carrying Amount			Fair Value		Carrying Amount		Fair Value
	Cash Convertible Senior Subordinated Notes	$	117,259		$	116,819	$	117,259	$	118,432

	Senior Secured Notes	305,000			309,194		305,000		320,823

	First Lien Credit Facility	—			—		235,000		235,000

	Total Debt	$	422,259		$	426,013	$	657,259	$	674,255

Debt Instrument Redemption	On or after June 15, 2015, KCG may redeem all or a part of the 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 under the Senior Secured Notes Registration Rights Agreement (defined below), if any, on the Senior Secured Notes redeemed, to the applicable redemption date, if redeemed during the 12-month period beginning on June 15 of the years indicated below:


	Year		Percentage
	2015		104.125	%
	2016		102.063	%
	2017 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 year ended December 31,
		2014			2013		2012
	Interest expense	$	31,724		$	34,138	$	1,194

	Amortization of debt issuance cost (1)	13,217			16,832		276

	Commitment fee	1,575			792		1,034

	Total	$	46,516		$	51,762	$	2,504


	(1)	Of the $13.2 million of amortization of debt issuance cost incurred during 2014, $9.6 million is included in Writedown of debt issuance costs and $3.6 million is in Other expenses. The writedown amounts were incurred as a result of the $235.0 million repayment of the First Lien Credit Facility made in 2014. For 2013, $3.6 million is included in Other expense while $13.2 million is included in Writedown of debt issuance cost as a result of the $300.0 million repayment of the First Lien Credit Facility.

Related_Parties_Tables

Related Parties (Tables)	12 Months Ended
	Dec. 31, 2014
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 year ended December 31,
	Statements of Operations	2014			2013
	Revenues
	Commissions and fees	$	14,528		$	4,045

	Trading revenues, net	3,862			437

	Interest, net	651			71

	Total revenues from related parties	$	19,041		$	4,553

	Expenses
	Execution and clearance fees(1)	$	(10,261	)	$	(19,491	)
	Payment for order flow	585			—

	Collateralized financing interest	529			102

	Other expense	1,719			651

	Total expenses incurred with respect to related parties	$	(7,428	)	$	(18,738	)
	(1) Represents net volume based fees received from providing liquidity to related trading venues.


	Statements of Financial Condition	December 31,			December 31,
		2014			2013
	Assets
	Securities borrowed	$	26,110		$	57,732

	Receivable from brokers, dealers and clearing organizations	20,075			20,826

	Other assets	—			277

	Liabilities
	Securities loaned	$	7,376		$	116,062

	Payable to brokers, dealers and clearing organizations	8,509			17,911

	Accrued expenses and other liabilities	5,667			179

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation [Abstract]
Compensation Expense Relating to Restricted Awards	Compensation expense from continuing operations relating to restricted awards, which is primarily recorded in Employee compensation and benefits, and the corresponding income tax benefit, which is recorded in Income tax (benefit) expense on the Consolidated Statements of Operations are presented in the following table (in thousands):


		For the year ended December 31,
		2014			2013
	Stock award compensation expense	$	55,402		$	33,067

	Income tax benefit	21,053			11,573

	The following is a summary of the changes in the incentive units for the year ended December 31, 2014 (units in thousands):


		Vested
	Incentive units at December 31, 2013	49

	Issued	—

	Vested	—

	Exercised	(11	)
	Canceled	—

	Incentive units at December 31, 2014	38

Summary of Restricted Awards Activity	The following table summarizes restricted awards activity for the year ended December 31, 2014 (awards in thousands):


			Restricted Stock Units
			Number of		Weighted-
			Units		Average
					Grant date
					Fair Value
	Outstanding at December 31, 2013		8,420		$	10.71

	Granted		4,937		10.99

	Vested		(3,464	)	10.77

	Forfeited		(746	)	11.66

	Outstanding at December 31, 2014		9,147		$	10.77

Stock Options, Valuation Assumptions	The weighted average assumptions used for stock options granted in 2013 were as follows:


		2013
	Dividend yield	—	%
	Expected volatility	35	%
	Risk-free interest rate	1	%
	Expected life (in years)	3.5

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 (benefit) expense on the Consolidated Statements of Operations are as follows (in thousands):


		For the year ended December 31,
		2014			2013
	Stock option and SAR compensation expense	$	3,807		$	1,813

	Income tax benefit	1,447			635

Summary of Stock Option Activity	The following table summarizes stock option and SAR activity and stock options exercisable for the year ended December 31, 2014 (awards in thousands):


			Number of Stock Awards		Weighted-				Aggregate			Weighted-
					Average				Intrinsic			Average
					Exercise				Value			Remaining
					Price							Life (years)
	Outstanding at December 31, 2013 (1)		4,967		$	18.45

	Granted at market value		—		—

	Exercised		—		—

	Forfeited or expired		(276	)	36.16

	Outstanding at December 31, 2014 (1)		4,691		$	17.41			$	3,438		3.41

	Exercisable at December 31, 2014		1,809		$	20.95			$	1,146		3.23

	Available for future grants at December 31, 2014 (2)		16,291

	(1) Includes 1.7 million of SARs.
	(2) Represents options, SARs and awards available for grant under the KCG Plan.
Shares Authorized under Stock Option Plans, by Exercise Price Range

			Options Outstanding								Options Exercisable
	Range of Exercise Prices		Outstanding		Weighted-		Weighted-				Number		Weighted-
			at 12/31/14		Average		Average				Exercisable		Average
					Remaining		Exercise				at 12/31/14		Exercise
					Contractual		Price						Price
					Life
	$8.24 - $8.24		833		3.61		$	8.24			278		$	8.24

	$8.25- $8.25		91		3.64		8.25				30		8.25

	$11.65 - $11.65		1,700		3.51		11.65				567		11.65

	$22.50 - $22.50		1,700		3.51		22.5				567		22.5

	$23.70 - $52.98		294		2.01		40.94				294		40.94

	$53.91 - $53.91		74		2.08		53.91				74		53.91

			4,691		3.41		$	17.41			1,809		$	20.95

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


		For the year ended December 31,
		2014			2013				2012
	Class B and E units	$	—		$	19,860			$	12,320

	Incentive units	(269		)	2,446				1,264

	Total	$	(269	)	$	22,306			$	13,584

Employee_Benefit_Plan_Tables

Employee Benefit Plan (Tables)	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Schedule of Defined Benefit Plans Disclosures	The total expense recognized with respect to the 401(k) Plan is included in Employee compensation and benefits on the Consolidated Statements of Operations, as follows (in thousands):


		For the year ended December 31,
		2014		2013
	Total expense	$	10,093	$	2,959

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Components of Provision (Benefit) for Income Taxes	The provision (benefit) for income taxes from continuing operations consists of (in thousands):


		For the year ended December 31,
		2014			2013			2012
	Current:
	U.S. federal	$	709		$	(194	)	$	(627	)

	U.S. state and local	4,081			2,902			618

	Non U.S.	(828		)	3,773			8,708

		$	3,962		$	6,481		$	8,699

	Deferred:
	U.S. federal	$	30,331		(106,996		)	1,771

	U.S. state and local	(10,997		)	—			1,046

	Non U.S.	(543		)	(599		)	(1,240		)
		$	18,791		$	(107,595	)	$	1,577


	Provision (benefit) for income taxes	$	22,753		$	(101,114	)	$	10,276

Income Tax Rate Reconciliation	The following table reconciles the U.S. federal statutory income tax to the Company's actual income tax from continuing operations (in thousands):


		For the year ended December 31,
		2014			2013			2014

	U.S. federal statutory income tax expense	$	29,815		$	8,742		$	9,249

	Income not subject to U.S. corporate income tax	—			(15,583		)	(7,095		)

	U.S. state and local income taxes, net of U.S. federal income tax effect	(4,495		)	1,881			772

	Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	—			(103,499		)	—

	Nondeductible expenses (1)	230			3,627			200

	Foreign taxes	(1,371		)	3,603			7,468

	Federal research & development tax credits	(1,241		)	—			—

	Other, net	(185		)	115			(318		)

	Income tax expense (benefit)	$	22,753		$	(101,114	)	$	10,276

	(1) Nondeductible expenses include nondeductible compensation and meals and entertainment.

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Accumulated Other Comprehensive Income	The following table presents changes in Accumulated other comprehensive income, net of tax by component for the years ended December 31, 2014, 2013 and 2012 (in thousands):


		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments		Total
	Balance January 1, 2012	$	—		$	—	$	—

	Other comprehensive income	114,319			—		114,319

	Balance December 31, 2012	114,319			—		114,319

	Other comprehensive income	13,689			1,365		15,054

	Amount reclassified from Accumulated Other Comprehensive Income (1)	(127,972		)	—		(127,972		)

	Net current-period other comprehensive (loss) income	(114,283		)	1,365		(112,918		)

	Balance December 31, 2013	36			1,365		1,401

	Other comprehensive income	316			416		732

	Balance December 31, 2014	$	352		$	1,781	$	2,133

	(1) As a result of the Mergers, the Company recorded a non-cash gain of $128.0 million on its investment in Knight Common Stock, which it recognized within Investment income and other, net on the Consolidated Statements of Operations for the year ended December 31, 2013 and reversed any previous unrealized gains out of Other comprehensive income.

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2014
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 year ended December 31, 2014, 2013 and 2012 (in thousands):


		For the year ended December 31,
		2014				2013					2012
		Numerator /		Denominator /		Numerator /			Denominator /		Numerator /		Denominator /
		net income		shares		net income			shares		net income		shares
	Income from continuing operations and shares used in basic calculations	$	62,434	112,854		$	126,092		80,143		16,151		48,970

	Effect of dilutive stock based awards
	Restricted awards			3,579					855				—

	Stock options and SARs			101					17				—

	Income from continuing operations and shares used in diluted calculations	$	62,434	116,534		$	126,092		81,015		$	16,151	48,970

	Loss from continuing operations allocated to preferred and participating units	$	—			$	(21,565	)			$	1,092

	Income from continuing operations attributable to common shareholders	$	62,434			$	147,657				$	15,059

	Basic earnings per common share from continuing operations			$	0.55				$	1.84			$	0.31

	Diluted earnings per common share from continuing operations			$	0.54				$	1.82			$	0.31

Commitments_and_Contingent_Lia1

Commitments and Contingent Liabilities (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule of Lease and Contract Obligations	The future minimum payments including interest under the capitalized leases at December 31, 2014 consist of (in thousands):


		Minimum Payments
	2015	$	4,223

	2016	2,126

	2017	620

	Total	$	6,969

Schedule of Interest Expense, Capital Lease	The total interest expense related to capital leases for the years ended December 31, 2014, and 2013 included in the Consolidated Statements Operations is as follows (in thousands):


		For the year ended December 31,
		2014		2013		2012
	Interest expense - Capital leases	$	370	$	700	$	1,376

Schedule of Future Minimum Rental Payments for Operating Leases	As of December 31, 2014, 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
	Year ending December 31, 2015	$	28,224	$	4,056	$	24,168

	Year ending December 31, 2016	27,762		3,488		24,274

	Year ending December 31, 2017	26,211		2,983		23,228

	Year ending December 31, 2018	25,354		2,667		22,687

	Year ending December 31, 2019	23,316		2,389		20,927

	Thereafter through December 31, 2027	64,441		10,244		54,197

	Total	$	195,308	$	25,827	$	169,481

Net_Capital_Requirements_Table

Net Capital Requirements (Tables)

12 Months Ended

Dec. 31, 2014

Brokers and Dealers [Abstract]

Net Capital Levels and Requirements for U.S. Registered Broker-Dealer Subsidiary

Entity

Net Capital

Excess Net

Requirement

Capital

KCG Americas LLC

285,231

1,000

284,231

Financial Resource Requirements on Significant Foreign Regulated Broker-Dealer

The following table sets forth the financial resource requirement for the following foreign regulated broker dealers at December 31, 2014 (in thousands):

Entity

Financial

Resource

Excess

Resources

Requirement

Financial

Resources

KCG Europe Limited

187,940

124,577

63,363

GETCO Europe Limited

7,254

1,759

5,495

Business_Segments_Tables

Business Segments (Tables)	12 Months Ended
	Dec. 31, 2014
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 year ended December 31, 2014:
	Revenues	$	901,152	$	345,710		$	69,369		$	1,316,232

	Pre-tax earnings	146,713		11,056			(72,582		)	85,187

	Total assets	4,401,021		786,734			1,642,899			6,830,654

	For the year ended December 31, 2013:
	Revenues	$	688,197	$	197,765		$	141,374		$	1,027,336

	Pre-tax earnings	103,612		(25,794		)	(52,842		)	24,978

	Total assets	3,939,965		1,106,448			1,950,591			6,997,004

	For the year ended December 31, 2012:
	Revenues	$	495,427	$	36,211		$	19,596		$	551,234

	Pre-tax earnings	34,887		(7,330		)	(1,130		)	26,427

	Total assets	1,408,516		33,623			245,397			1,687,536

	The Company operates in the U.S. and internationally, primarily in Europe and Asia. The following table presents Revenues by geographic area.


		U.S.		International			Consolidated
							Total
	For the year ended December 31, 2014:
	Revenues	$	1,127,088	$	189,144		$	1,316,232

	For the year ended December 31, 2013:
	Revenues	$	834,410	$	192,926		$	1,027,336

	For the year ended December 31, 2012:
	Revenues	$	340,117	$	211,117		$	551,234

Condensed_Financial_Statements1

Condensed Financial Statements of KCG Holdings, Inc. (parent only) (Tables)	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Statements of Financial Condition Parent Only	Presented below are the Condensed Statements of Financial Condition, Operations and Cash Flows for the Company on an unconsolidated basis.


	Statements of Financial Condition
	KCG Holdings, Inc. (parent only)


		December 31,
		2014				2013
		(in thousands)
	Assets
	Cash and cash equivalents	$	302,700			$	253,017

	Receivable from subsidiaries	21,851				60,460

	Investments in subsidiaries	1,122,254				1,359,779

	Income taxes receivable	13,999				90

	Deferred tax asset, net	137,009				143,028

	Subordinated loans to subsidiaries	280,000				250,000

	Other assets	19,397				21,182

	Total assets	$	1,897,210			$	2,087,556

	Liabilities and equity
	Liabilities
	Accrued compensation expense	$	13,208			$	—

	Payable to subsidiaries	42,478				25,705

	Accrued expenses and other liabilities	13,947				12,314

	Debt	305,000				540,000

	Total liabilities	374,633				578,019

	Total equity	1,522,577				1,509,537

	Total liabilities and equity	$	1,897,210			$	2,087,556

Statements of Operations Parent Only	Statements of Operations
	KCG Holdings, Inc. (parent only)


		For the year ended December 31,
		2014				2013				2012
		(in thousands)
	Revenues
	Investment income and other, net	$	3,415			$	3,717			$	5,981

	Total revenues	3,415				3,717				5,981

	Expenses
	Employee compensation and benefits	50,256				31,970				(12,251		)

	Interest	11,939				28,476				1,195

	Depreciation and amortization	—				698				1,395

	Professional fees	9,211				30,488				4,997

	Business development	3,625				—				—

	Other	25,460				29,829				1,493

	Total expenses	100,491				121,461				(3,171		)

	(Loss) income before income taxes and equity in earnings of subsidiaries	(97,076		)		(117,744		)		9,152

	Income tax benefit	(35,972		)		(120,761		)		—

	Income (loss) before equity in earnings of subsidiaries	(61,104		)		3,017				9,152

	Equity in earnings of subsidiaries	122,206				123,155				6,999

	Net income	$	61,102			$	126,172			$	16,151

Statements of Cash Flows Parent Only	Statements of Cash Flows
	KCG Holdings, Inc. (parent only)


		For the year ended December 31,
		2014				2013				2012
		(In thousands)
	Cash flows from operating activities
	Net income	$	61,102			$	126,172			$	16,151

	Adjustments to reconcile net income to net cash used in operating activities
	Equity in earnings of subsidiaries	(122,206		)		(123,155		)		(6,999		)
	Writedown and amortization of debt issuance costs	12,548				16,931				52

	Amortization of intangibles	—				698				1,395

	Stock-based compensation	16,997				—				—

	Decrease (increase) in operating assets
	Subordinated loan receivable	(30,000		)		(250,000		)		—

	Receivable from subsidiaries	38,609				(67,523		)		(97,375		)

	Deferred tax asset	(13,909		)		(144,109		)		—

	Income taxes receivable	6,019				(90		)		—

	Other assets	(10,763		)		(15,351		)		31,031

	(Decrease) increase in operating liabilities
	Payable to subsidiaries	16,773				(2,024		)		11,756

	Accrued compensation expense	8,839				—				—

	Accrued expenses and other liabilities	1,633				59,561				5,106

	Net cash used in operating activities	(14,358		)		(398,890		)		(38,883		)
	Cash flows from investing activities
	Cash acquired upon acquisition of Knight Capital Group, Inc.	—				509,133				—

	Cash received from sale of Urban Financial of America, LLC	—				85,406				—

	Dividends received from subsidiaries	807,715				646,425				161,606

	Capital contributions to subsidiaries	(397,089		)		(238,909		)		(70,794		)
	Net cash provided by investing activities	410,626				1,002,055				90,812

	Cash flows from financing activities
	Proceeds from issuance of Credit Agreement	—				535,000				—

	Partial repayment of Credit Agreement	(235,000		)		(300,000		)		—

	Proceeds from issuance of Senior Secured Notes	—				305,000				—

	Repayment of notes payable	—				(15,000		)		—

	Payment of debt issuance costs	—				(34,592		)		—

	Issuance of equity to General Atlantic	—				55,000				—

	Payment to former Knight Capital Group, Inc. stockholders	—				(720,000		)		—

	Repayment of Knight Convertible Notes	—				(257,741		)		—

Funding of collateral account for Knight Convertible Notes	—				(117,259		)		—

Payment out of collateral account for Knight Convertible Notes	—				117,259				—

Proceeds from stock issuance	—				—				12,320

Member distributions	—				(21,002		)		(62,234		)

Cost of common stock repurchased	(111,585		)		(11,324		)		—

Net cash used in financing activities	(346,585		)		(464,659		)		(49,914		)
Effect of exchange rate changes on cash and cash equivalents	—				—				—

Increase in cash and cash equivalents	49,683				138,506				2,015

Cash and cash equivalents at beginning of period	253,017				114,511				112,496

Cash and cash equivalents at end of period	$	302,700			$	253,017			$	114,511

Supplemental disclosure of cash flow information:

Cash paid for interest	$	28,426			$	26,239			$	1,116

Cash paid for income taxes	$	15,456			$	365			$	—

Merger_of_GETCO_and_Knight_Det

Merger of GETCO and Knight (Detail) (USD $)	0 Months Ended	6 Months Ended
Share data in Millions, except Per Share data, unless otherwise specified	Jul. 01, 2013	Jun. 30, 2014	Jul. 01, 2013
Merger Transaction [Line Items]
Right of Knight shareholders to elect to receive per share value in cash under the agreement (in dollars per share)	$3.75
Maximum cash entitled by shareholders to receive under the agreement	$720,000,000
Shares granted to former shareholders (in shares)	41.9
Knight [Member]
Merger Transaction [Line Items]
Total purchase price	1,370,000,000
Goodwill adjustments		1,100,000
Deferred tax assets adjustments		3,100,000
Accrued expenses adjustments		3,500,000
Other assets adjustments		700,000
Deferred tax asset, net	$65,465,000		$65,465,000
Warrants one [Member]
Merger Transaction [Line Items]
Warrants receivable from holding company (in shares)	8.1
Exercise price of warrant (in dollars per share)	$12		$12
Term of warrant	4 years
Warrants two [Member]
Merger Transaction [Line Items]
Warrants receivable from holding company (in shares)	8.1
Exercise price of warrant (in dollars per share)	$13.50		$13.50
Term of warrant	5 years
Warrants three [Member]
Merger Transaction [Line Items]
Warrants receivable from holding company (in shares)	8.1
Exercise price of warrant (in dollars per share)	$15		$15
Term of warrant	6 years
GETCO [Member]
Merger Transaction [Line Items]
Number of shares to be received by GETCO members (in shares)	75.9
Number of warrants to acquire shares of common stock (in shares)	24.3		24.3
Knight Class A Common Stock [Member]
Merger Transaction [Line Items]
Par value (in dollars per share)	$0.01		$0.01
KCG Class A Common Stock [Member]
Merger Transaction [Line Items]
Par value (in dollars per share)	$0.01		$0.01

Merger_of_GETCO_and_Knight_Ass

Merger of GETCO and Knight - Assets Acquired and Liabilities Assumed (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Jul. 01, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Jul. 01, 2013
Identifiable Net Assets
Goodwill	$17,311	$17,311
Knight [Member]
Identifiable Net Assets
Cash and cash equivalents			509,133
Cash and cash equivalents segregated under federal and other regulations			203,045
Financial instruments owned			1,937,929
Securities borrowed			1,158,981
Receivable from brokers, dealers and clearing organizations			1,369,474
Fixed assets and leasehold improvements			80,280
Investments			106,353
Intangible assets			155,425
Assets within discontinued operations			5,607,063
Deferred tax asset, net			65,465
Other assets			140,933
Total Assets			11,334,081
Financial instruments sold, not yet purchased			1,512,983
Collateralized financings			1,166,211
Payable to brokers, dealers and clearing organizations			635,914
Payable to customers			527,918
Accrued compensation expense			107,409
Accrued expenses and other liabilities			130,010
Liabilities within discontinued operations			5,518,168
Debt			375,000
Total Liabilities			9,973,613
Total identified assets acquired, net of assumed liabilities			1,360,468
Goodwill			12,666
Total Purchase Price			$1,373,134

Merger_of_GETCO_and_Knight_Int

Merger of GETCO and Knight - Intangible Assets Acquired (Details) (USD $)	12 Months Ended		0 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Jul. 01, 2013
Business Acquisition [Line Items]
Goodwill	$17,311	$17,311
Weighted average useful life	5 years	6 years
Knight [Member]
Business Acquisition [Line Items]
Intangible assets			155,425
Goodwill			12,666
Total			168,091
Knight [Member] \| Technology [Member]
Business Acquisition [Line Items]
Intangible assets			110,504
Weighted average useful life			5 years
Knight [Member] \| Customer Relationships [Member]
Business Acquisition [Line Items]
Intangible assets			35,000
Knight [Member] \| Customer Relationships [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Weighted average useful life			9 years
Knight [Member] \| Customer Relationships [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Weighted average useful life			11 years
Knight [Member] \| Trade Names [Member]
Business Acquisition [Line Items]
Intangible assets			4,000
Weighted average useful life			10 years
Knight [Member] \| Trading Rights [Member]
Business Acquisition [Line Items]
Intangible assets			$5,921	[1]
Weighted average useful life			7 years	[1]

[1]	Trading rights include both assets with a finite useful life and assets with an indefinite useful life. The 7 years amortization period only applies to assets with a finite useful life.

Significant_Accounting_Policie3

Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Increase in investments	$100,726,000	$125,413,000
Decrease in deferred tax assets, net	-222,631,000	-234,015,000
Increase in retained earnings	272,780,000	211,678,000
Maximum maturity of short-term investments	90 days
Minimum [Member]
Property, Plant and Equipment [Line Items]
Amortization period, in years	1 year
Fixed assets, useful life	3 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Amortization period, in years	9 years
Fixed assets, useful life	7 years
Software [Member]
Property, Plant and Equipment [Line Items]
Amortization period, in years	3 years
Investment Income and Other, Net [Member]
Property, Plant and Equipment [Line Items]
Non-cash gain on investment resulting from the merger		128,000,000
Foreign Currency Transactions [Member]
Property, Plant and Equipment [Line Items]
Investment gains (losses)	-2,000,000	-4,100,000	200,000
BATS and DE [Member] \| Change in Ownership Interest [Member]
Property, Plant and Equipment [Line Items]
Increase in investments		3,400,000
Decrease in deferred tax assets, net		1,100,000
Increase in retained earnings		2,300,000
Increase in investment income and other, net		7,200,000
Increase in net income		$6,100,000

Significant_Accounting_Policie4

Significant Accounting Policies - Schedule of Interest Income and Interest Expense (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Interest Income	$14,363	$10,187	$3,070
Interest Expense	-13,742	-10,724	-5,427
Interest, net	$621	($537)	($2,357)

Significant_Accounting_Policie5

Significant Accounting Policies - Net Trading Revenue Including Dividend Income and Expense (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Dividend Income	$45,910	$26,237	$2,253
Dividend Expense	($38,444)	($21,630)	($1,599)

Discontinued_Operations_Assets2

Discontinued Operations & Assets and LIabilities Held for Sale - Income Disclosures (Detail) (USD $)	12 Months Ended			6 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013
Expenses:
Compensation	$437,269	$349,192	$157,855
Payments for order flow	70,183	35,711	2,964
Other expense	39,814	43,094	23,073
Total expenses	1,231,045	1,002,358	524,807
Loss (Income) from discontinued operations, net of tax	-1,332	80	0
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenues & Gain on sale	-1,148			39,868
Expenses:
Compensation	70			14,068
Payments for order flow	0			9,885
Execution and clearance fees	0			5,038
Other expense	930			10,748
Total expenses	1,000			39,739
Pre-tax (loss) income from discontinued operations	-2,148			129
Income tax benefit (expense)	816			-49
Loss (Income) from discontinued operations, net of tax	($1,332)			$80

Discontinued_Operations_Assets3

Discontinued Operations & Assets and LIabilities Held for Sale - Balance Sheet Disclosures (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Liabilities:
Total liabilities of business held for sale	$2,356	$0
FCM [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

Assets_Segregated_or_Held_in_S2

Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations - (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Regulated Operations [Abstract]
Cash and cash equivalents segregated under federal or other regulations	$3,361		$183,082
Receivables from brokers, dealers and clearing organizations	0	[1]	304,294	[1]
Total assets segregated or held in separate accounts under federal or other regulations	$3,361		$487,376

[1]	Segregated assets included within Receivable from brokers, dealers and clearing organizations comprise cash and cash equivalents and U.S. government obligations primarily held as deposits with exchange clearing organizations.

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	$2,707,371,000		$2,721,839,000
Financial instrument assets	2,712,820,000		2,898,074,000
Financial instruments sold, not yet purchased, at fair value	2,285,707,000		2,165,500,000
Total fair value of financial instrument liabilities	2,285,707,000		2,165,500,000
Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,479,910,000	[1]	2,298,785,000	[2]
Financial instruments sold, not yet purchased, at fair value	2,069,342,000	[1]	1,851,006,000	[2]
Securities netted by respective long and short positions	743,100,000		697,900,000
Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	144,586,000		339,798,000
Financial instruments sold, not yet purchased, at fair value	115,362,000		252,282,000
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	22,983,000		40,053,000
Corporate Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	59,832,000	[3]	43,203,000
Financial instruments sold, not yet purchased, at fair value	82,050,000	[3]	42,122,000
Securities netted by respective long and short positions	300,000
Securities on Deposit with Clearing Organizations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities on deposit with clearing organizations			170,235,000	[4]
Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	4,435,000	[5]	3,925,000	[5]
Deferred Compensation Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	868,000	[5]	117,000	[5]
Investment In Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	146,000	[5]	1,958,000	[5]
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	18,953,000		15,076,000
Foreign Currency Forward Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	60,000
Financial instruments sold, not yet purchased, at fair value			5,014,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,707,311,000		2,721,839,000
Financial instrument assets	2,711,746,000		2,895,999,000
Total fair value of financial instrument liabilities	2,285,707,000		2,160,486,000
Level 1 [Member] \| Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,479,910,000	[1]	2,298,785,000	[2]
Financial instruments sold, not yet purchased, at fair value	2,069,342,000	[1]	1,851,006,000	[2]
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	144,586,000		339,798,000
Financial instruments sold, not yet purchased, at fair value	115,362,000		252,282,000
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	22,983,000		40,053,000
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	59,832,000	[3]	43,203,000
Financial instruments sold, not yet purchased, at fair value	82,050,000	[3]	42,122,000
Level 1 [Member] \| Securities on Deposit with Clearing Organizations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities on deposit with clearing organizations			170,235,000	[4]
Level 1 [Member] \| Investment in CME Group [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	4,435,000	[5]	3,925,000	[5]
Level 1 [Member] \| Deferred Compensation Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	0	[5]	0	[5]
Level 1 [Member] \| Investment In Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	0	[5]	0	[5]
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	18,953,000		15,076,000
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	60,000		0
Financial instrument assets	1,074,000		2,075,000
Total fair value of financial instrument liabilities	0		5,014,000
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	[1]	0	[2]
Financial instruments sold, not yet purchased, at fair value	0	[1]	0	[2]
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	[3]	0
Financial instruments sold, not yet purchased, at fair value	0	[3]	0
Level 2 [Member] \| Securities on Deposit with Clearing Organizations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities on deposit with clearing organizations			0	[4]
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	[5]	0	[5]
Level 2 [Member] \| Deferred Compensation Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	868,000	[5]	117,000	[5]
Level 2 [Member] \| Investment In Deephaven Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instrument assets	146,000	[5]	1,958,000	[5]
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	60,000
Financial instruments sold, not yet purchased, at fair value			$5,014,000

[1]	Equities of $743.1 million have been netted by their respective long and short positions by CUSIP number.
[2]	Equities of $697.9 million have been netted by their respective long and short positions by CUSIP number.
[3]	Corporate debt instruments of $0.3 million have been netted by their respective long and short positions by CUSIP number.
[4]	Securities on deposit with clearing organizations consist of U.S. government obligations and are recorded within Receivable from brokers, dealers and clearing organizations on the Consolidated Statements of Financial Condition.
[5]	Investment in CME Group, Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition.

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments - Additional Information (Detail)	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2013
	USD ($)	INR	USD ($)	GBP (£)
Fair Value Disclosures [Abstract]
Transfers of financial instruments between levels	$0		$0
Transfers of financial instruments between level 2 to level 1	0		0
Transfers of financial instruments out of level 3	0		0
Notional value of foreign currency forward	$10,900,000	700,000,000		£ 80,000,000

Derivative_Financial_Instrumen2

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

[1]	Designated as hedging instrument.

Derivative_Financial_Instrumen3

Derivative Financial Instruments - Gain (Loss) Recognized (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
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)	$90,718		$253,761		$272,235
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)	10,535		12,191		19,699
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)	526		0		0
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)	25,247		50,073		76,089
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)	5,277		11,736		8,116
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)	-37,439	[1]	37,035	[1]	50,446	[1]
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)	31,277		80,511		81,933
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)	55,295		62,215		35,952
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		($3,298)		$0

[1]	Realized gains and losses on listed equity options relate to the Companybs 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 Companybs Consolidated Statements of Operations.

Collateralized_Transactions_an2

Collateralized Transactions and Assets and Liabilities Subject to Netting - Financial Instruments at Fair Value Received as Collateral that were Permitted to be Delivered or Repledged (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Collateralized Agreements [Abstract]
Collateral permitted to be delivered or repledged	$1,586,700	$1,315,803
Collateral that was delivered or repledged	1,485,267	1,231,468
Collateral permitted to be further repledged by the receiving counterparty	$147,696	$142,938

Collateralized_Transactions_an3

Collateralized Transactions and Assets and Liabilities Subject to Netting - Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Repurchase Agreement Counterparty [Line Items]
Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge	$1,586,700	$1,315,803
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	536,124	552,242
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	$979,652	$676,956

Collateralized_Transactions_an4

Collateralized Transactions and Assets and Liabilities Subject to Netting - Gross Amounts Offset (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	$1,823,018		$1,736,983
Gross Amounts Offset in the Statements of Financial Condition, Assets	24,436		10,870
Net Amounts Presented in the Statements of Financial Condition, Assets	1,798,582		1,726,113
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	1,591,619	[1]	1,340,359	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	15,782	[2]	10,110	[2]
Net Amount, Assets	191,181		375,644
Gross Amounts of Recognized Derivatives, Liabilities	1,781,118		1,643,258
Gross Amounts of Recognized Derivatives, Liabilities	24,436		11,764
Net Amounts Presented in the Statements of Financial Condition, Liabilities	1,756,682		1,631,494
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	1,615,949	[1]	1,357,786	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	33,141	[2]	21,034	[2]
Net Amount, Liabilities	107,592		252,674
Listed Options [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	144,586		339,798
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		0
Net Amounts Presented in the Statements of Financial Condition, Assets	144,586		339,798
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]	0	[2]
Net Amount, Assets	144,586		339,798
Gross Amounts of Recognized Derivatives, Liabilities	115,362		252,282
Gross Amounts of Recognized Derivatives, Liabilities	0		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	115,362		252,282
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	17,359	[2]	10,924	[2]
Net Amount, Liabilities	98,003		241,358
Securities Borrowed [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	1,632,062		1,357,387
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		0
Net Amounts Presented in the Statements of Financial Condition, Assets	1,632,062		1,357,387
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	1,570,194	[1]	1,316,110	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	15,782	[2]	10,110	[2]
Net Amount, Assets	46,086		31,167
Receivable from Brokers, Dealers and Clearing Organizations [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	21,545	[3]	24,366	[3]
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	[3]	0	[3]
Net Amounts Presented in the Statements of Financial Condition, Assets	21,545	[3]	24,366	[3]
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	21,425	[1],[3]	24,249	[1],[3]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2],[3]	0	[2],[3]
Net Amount, Assets	120	[3]	117	[3]
Foreign Currency Forward Contracts [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	60		6,913
Gross Amounts Offset in the Statements of Financial Condition, Assets	0		6,501
Net Amounts Presented in the Statements of Financial Condition, Assets	60		412
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]	0	[2]
Net Amount, Assets	60		412
Gross Amounts of Recognized Derivatives, Liabilities			11,515
Gross Amounts of Recognized Derivatives, Liabilities			6,501
Net Amounts Presented in the Statements of Financial Condition, Liabilities			5,014
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities			0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities			0	[2]
Net Amount, Liabilities			5,014
Swaps [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	85
Gross Amounts Offset in the Statements of Financial Condition, Assets	0
Net Amounts Presented in the Statements of Financial Condition, Assets	85
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]
Net Amount, Assets	85
Gross Amounts of Recognized Derivatives, Liabilities			18
Gross Amounts of Recognized Derivatives, Liabilities			0
Net Amounts Presented in the Statements of Financial Condition, Liabilities			18
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities			0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities			0	[2]
Net Amount, Liabilities			18
Futures [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized, Assets	24,680		8,519
Gross Amounts Offset in the Statements of Financial Condition, Assets	24,436		4,369
Net Amounts Presented in the Statements of Financial Condition, Assets	244		4,150
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Assets	0	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Assets	0	[2]	0	[2]
Net Amount, Assets	244		4,150
Gross Amounts of Recognized Derivatives, Liabilities	24,436		5,263
Gross Amounts of Recognized Derivatives, Liabilities	24,436		5,263
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	[1]	0	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	[2]	0	[2]
Net Amount, Liabilities	0		0
Securities Loaned [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liabilities	707,744		733,230
Gross Amounts of Recognized Derivatives, Liabilities	0		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	707,744		733,230
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	682,389	[1]	716,838	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	15,782	[2]	10,110	[2]
Net Amount, Liabilities	9,573		6,282
Financial Instruments Sold Under Agreements to Repurchase [Member}
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Derivatives, Liabilities	933,576		640,950
Gross Amounts of Recognized Derivatives, Liabilities	0		0
Net Amounts Presented in the Statements of Financial Condition, Liabilities	933,576		640,950
Gross Amounts Not Offset in the Statement of Financial Condition, Available Collateral, Liabilities	933,560	[1]	640,948	[1]
Gross Amounts Not Offset in the Statement of Financial Condition, Counterparty Netting, Liabilities	0	[2]	0	[2]
Net Amount, Liabilities	$16		$2

[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 avalable 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 reverse repurchase agreements at broker dealer.

Collateralized_Transactions_an5

Collateralized Transactions and Assets and Liabilities Subject to Netting - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014
Collateralized Agreements [Abstract]
Repurchase agreements and other secured financings, maturity (years)	1 year

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 $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Receivable/Payable:
Clearing organizations and other	$1,095,025	$750,440
Assets segregated or held in separate accounts under federal or other regulations (1)	0	304,294
Securities failed to deliver	93,808	202,517
Total Receivable	1,188,833	1,257,251
Clearing organizations and other	350,627	425,196
Securities failed to receive	325,462	48,912
Total Payable	$676,089	$474,108

Fixed_Assets_and_Leasehold_Imp2

Fixed Assets and Leasehold Improvements - Schedule of Fixed Assets and Leasehold Improvements (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Fixed assets and leasehold improvements, gross	$373,431	$354,262
Less - Accumulated depreciation and amortization	-239,380	-207,594
Fixed assets and leasehold improvements, at cost, less accumulated depreciation and amortization	134,051	146,668
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Depreciation Period	3 years
Fixed assets and leasehold improvements, gross	242,498	223,141
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Fixed assets and leasehold improvements, gross	114,851	115,046
Telephone Systems and Equipment [Member]
Property, Plant and Equipment [Line Items]
Depreciation Period	5 years
Fixed assets and leasehold improvements, gross	3,962	3,848
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Depreciation Period	7 years
Fixed assets and leasehold improvements, gross	$12,120	$12,227

Investments_Details

Investments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investments [Abstract]
Investments accounted for under the equity method	$86,328	$110,460
Investments held at fair value	4,435	3,925
Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	8,949	8,953
Total Strategic investments	99,712	123,338
Deferred compensation investments	868	117
Investment in Deephaven Funds	146	1,958
Total Investments	$100,726	$125,413

Investments_Additional_Informa

Investments - Additional Information (Details) (USD $)	12 Months Ended			3 Months Ended
Investments - Additional Information (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Sep. 30, 2014
Schedule of Equity Method Investments [Line Items]
Proceeds and distributions from investments	$58,660,000	$3,251,000	$72,196,000
Investment income and other, net	41,232,000	124,095,000	27,010,000
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
tradeMonster [Member] \| Investment Income and Other, Net [Member]
Schedule of Equity Method Investments [Line Items]
Investment income and other, net					15,100,000
TM Holdings [Member]
Schedule of Equity Method Investments [Line Items]
Investment income and other, net					$17,600,000

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Jul. 01, 2013
Finite-Lived Intangible Assets [Line Items]
Goodwill	$17,311,000	$17,311,000	$17,311,000	$17,311,000
Goodwill and intangible assets impairment	0	0
Weighted average useful life			5 years	6 years
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period, in years			1 year
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period, in years			9 years
Knight [Member]
Finite-Lived Intangible Assets [Line Items]
Identifiable intangible assets					155,425,000
Goodwill					$12,666,000

Goodwill_and_Intangible_Assets3

Goodwill and Intangible Assets - Summary of Goodwill (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
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 $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$192,272,000		$221,796,000
Accumulated amortization	-56,989,000		-30,301,000
Net carrying amount	135,283,000		191,495,000
KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles	34,700,000
Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	115,804,000	[1]	120,346,000	[1]
Accumulated amortization	-36,562,000		-14,849,000
Net carrying amount	79,242,000		105,497,000
Amortization period, in years	3 years		4 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	[2]	62,450,000	[2]
Accumulated amortization	-18,110,000		-13,530,000
Net carrying amount	44,358,000		48,920,000
Amortization period, in years	7 years		8 years
Indefinite-lived intangible assets	6,800,000		7,600,000
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	13,000,000	[3]	35,000,000	[3]
Accumulated amortization	-2,167,000		-1,722,000
Net carrying amount	10,833,000		33,278,000
Amortization period, in years	8 years		10 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	[4]	4,000,000	[4]
Accumulated amortization	-150,000		-200,000
Net carrying amount	850,000		3,800,000
Amortization period, in years	9 years		10 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	94,900,000		102,235,000
Market Making [Member] \| Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	50,542,000		53,315,000
Market Making [Member] \| Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	44,358,000		48,920,000
Electronic Execution Services [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	29,883,000	[5]	75,760,000	[5]
Electronic Execution Services [Member] \| Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	18,200,000		38,682,000
Electronic Execution Services [Member] \| Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	10,833,000		33,278,000
Electronic Execution Services [Member] \| Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	850,000		3,800,000
Corporate and Other [Member] \| Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$10,500,000		$13,500,000

[1]	The weighted average remaining life for technology, including capitalized software, was approximately 3 and 4 years as of DecemberB 31, 2014 and DecemberB 31, 2013, respectively. Excluded from the December 31, 2014 balance is $13.1 million of technology assets related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", 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 and 8 years as of DecemberB 31, 2014 and DecemberB 31, 2013, respectively. As of DecemberB 31, 2014 and December 31, 2013, $6.8 million and $7.6 million, respectively, of trading rights had indefinite useful lives.
[3]	Customer relationships relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 8 and 10 years as of DecemberB 31, 2014 and DecemberB 31, 2013, 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 the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", is included in Assets of business held for sale at December 31, 2014.
[4]	Trade names relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 9 and 10 years as of DecemberB 31, 2014 and DecemberB 31, 2013, respectively. Excluded from the December 31, 2014 balance is $2.6 million of trade name related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", is included in Assets of business held for sale at December 31, 2014.
[5]	Excluded from the December 31, 2014 balance is $34.7 million of intangibles related to the KCG Hotspot which is held for sale. As noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", such amount is included in Assets of business held for sale at December 31, 2014.

Goodwill_and_Intangible_Assets5

Goodwill and Intangible Assets - Summary of Amortization Expense Relating to Intangible Assets (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense	$35,592	$19,211	$5,518

Goodwill_and_Intangible_Assets6

Goodwill and Intangible Assets - Summary of Estimated Amortization Expense for Future Years (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
For the year ended December 31, 2015	$31,180
For the year ended December 31, 2016	30,049
For the year ended December 31, 2017	27,244
For the year ended December 31, 2018	15,611
For the year ended December 31, 2019	$6,211

Debt_Summary_of_LongTerm_Debt_

Debt - Summary of Long-Term Debt (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt Disclosure [Abstract]
Cash Convertible Senior Subordinated Notes, Carrying Amount	$117,259	$117,259
Senior Secured Notes, Carrying Amount	305,000	305,000
First Lien Credit Facility, Carrying Amount	0	235,000
Total Debt, Carrying Amount	422,259	657,259
Cash Convertible Senior Subordinated Notes, Fair Value	116,819	118,432
Senior Secured Notes, Fair Value	309,194	320,823
First Lien Credit Facility, Fair Value	0	235,000
Total Debt, Fair Value	$426,013	$674,255

Debt_Debt_Redemption_Details

Debt - Debt Redemption (Details) (Secured Debt [Member])	0 Months Ended
Debt - Debt Redemption (Details) (Secured Debt [Member])	Jun. 05, 2014
Debt Instrument [Line Items]
Percentage	108.25%
2015
Debt Instrument [Line Items]
Percentage	104.13%
2016
Debt Instrument [Line Items]
Percentage	102.06%
2017 and thereafter
Debt Instrument [Line Items]
Percentage	100.00%

Debt_Recorded_Expenses_with_Re

Debt - Recorded Expenses with Respect to Long-term Debt (Details) (USD $)	12 Months Ended						6 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2013
Line of Credit Facility [Line Items]
Interest expense	$31,724,000		$34,138,000		$1,194,000
Amortization of debt issuance cost	13,217,000	[1]	16,832,000	[1]	276,000	[1]
Commitment fee	1,575,000		792,000		1,034,000
Total	46,516,000		51,762,000		2,504,000
Writedown of debt issuance costs	9,552,000		13,209,000		0
First Lien Credit Facility [Member]
Line of Credit Facility [Line Items]
Writedown of debt issuance costs							13,200,000
Jefferies Finance LLC and Goldman Sachs Bank USA [Member] \| First Lien Credit Facility [Member]
Line of Credit Facility [Line Items]
Writedown of debt issuance costs	9,600,000
Early principal payments	235,000,000		300,000,000				300,000,000
Writedown of Debt Issuance Costs [Member]
Line of Credit Facility [Line Items]
Writedown of debt issuance costs	9,600,000		13,200,000
Other Expense [Member]
Line of Credit Facility [Line Items]
Amortization of debt issuance cost	$3,600,000		$3,600,000

[1]	Of the $13.2 million of amortization of debt issuance cost incurred during 2014, $9.6 million is included in Writedown of debt issuance costs and $3.6 million is in Other expenses. The writedown amounts were incurred as a result of the $235.0 million repayment of the First Lien Credit Facility made in 2014.

Debt_Additional_Information_De

Debt - Additional Information (Detail) (USD $)	0 Months Ended	1 Months Ended	12 Months Ended			6 Months Ended	0 Months Ended
Debt - Additional Information (Detail) (USD $)	Jul. 01, 2013	Mar. 31, 2010	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013	Jun. 05, 2014	31-May-14	Oct. 31, 2013	Jun. 05, 2013
Line of Credit Facility [Line Items]
Issue of cash convertible senior subordinated notes		$375,000,000
Notes bear interest rate per year		3.50%
Date of commencing of notes		15-Sep-10
Deposit cash made to credit facility to repurchase convertible notes	375,000,000
Repayment of Knight Convertible Notes				257,700,000
Interest payable				3,600,000		3,600,000
Convertible debt			117,259,000	117,259,000		117,259,000
Convertible Notes, conversion rate (in shares)			0.0159728
Convertible Notes, conversion price (in dollars per share)			$62.61
Fair value of embedded conversion derivative			0	0		0
Term credit agreement			0	235,000,000		235,000,000
Cash held in collateral account				117,300,000		117,300,000
Writedown of capitalized debt costs			9,552,000	13,209,000	0
Debt issuance costs incurred			38,500,000
Secured Debt [Member]
Line of Credit Facility [Line Items]
Interest rate										8.25%
Term credit agreement										305,000,000
Escrow agent fees and expenses	3,000,000
Number of days to issue securities	365 days
Additional interest percent obligation from guarantor if certain obligations are not met	1.00%
Amount of consenting shareholders percentage of total debt								98.50%	99.70%
Early principal payments			235,000,000	300,000,000
Writedown of capitalized debt costs			9,600,000	13,200,000
Interest rate at end of period			9.10%
First Lien Credit Facility [Member]
Line of Credit Facility [Line Items]
Writedown of capitalized debt costs						13,200,000
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
Early principal payments			235,000,000	300,000,000		300,000,000
Writedown of capitalized debt costs			9,600,000
Percentage points added to interest rate base	4.50%
First Lien Credit Facility [Member] \| Jefferies Finance LLC and Goldman Sachs Bank USA [Member] \| Prime Rate [Member]
Line of Credit Facility [Line Items]
Interest rate of credit agreement	2.25%
Percentage points added to interest rate base	0.50%
First Lien Credit Facility [Member] \| Jefferies Finance LLC and Goldman Sachs Bank USA [Member] \| LIBOR [Member]
Line of Credit Facility [Line Items]
Interest rate of credit agreement	1.00%
Percentage points added to interest rate base	3.50%
First Lien Credit Facility [Member] \| Jefferies Finance LLC and Goldman Sachs Bank USA [Member] \| LIBOR Floor [Member]
Line of Credit Facility [Line Items]
Interest rate of credit agreement	1.25%
Revolving Credit Facility [Member] \| OCTEG-KCA [Member]
Line of Credit Facility [Line Items]
Term credit agreement	450,000,000		0
Revolving credit facility amount	750,000,000
Debt issuance costs incurred			1,200,000
Number of debt instruments	2
Number of borrowing base	2
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%
Revolving Credit Facility [Member] \| Borrowing Base A [Member] \| OCTEG-KCA [Member]
Line of Credit Facility [Line Items]
Percentage points added to interest rate base	1.75%
Revolving Credit Facility [Member] \| Borrowing Base B Loan [Member] \| OCTEG-KCA [Member]
Line of Credit Facility [Line Items]
Percentage points added to interest rate base	2.25%
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
Swingline Facilty [Member] \| OCTEG-KCA [Member]
Line of Credit Facility [Line Items]
Revolving credit facility amount	$50,000,000
Convertible Notes [Member]
Line of Credit Facility [Line Items]
Redemption price percentage	100.00%
Secured Debt [Member]
Line of Credit Facility [Line Items]
Redemption price percentage							108.25%
Redemption period							12 months
Maximum redemption percentage of principal amount							35.00%
Secured Debt [Member] \| Prior to June 15, 2015 [Member]
Line of Credit Facility [Line Items]
Redemption price percentage							100.00%
Scenario, Actual [Member]
Line of Credit Facility [Line Items]
Debt maturity date		15-Mar-15
Date of mature of notes		15-Mar-15
Minimum [Member] \| Secured Debt [Member]
Line of Credit Facility [Line Items]
Redemption notice, period							30 days
Maximum [Member] \| Secured Debt [Member]
Line of Credit Facility [Line Items]
Redemption notice, period							60 days

Related_Parties_Additional_Inf

Related Parties - Additional Information (Details) (Related Parties [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2014
	Dec. 31, 2013	investee
Related Party Transaction [Line Items]
Number of equity method investees		3
Financing and advisory fees	$49.80
Minimum [Member] \| Common Class A [Member]
Related Party Transaction [Line Items]
Number of beneficial owners		2
Noncontrolling interest, ownership percentage by noncontrolling owners		10.00%

Related_Parties_Summary_of_Bal

Related Parties - Summary of Balances and Transactions with Related Parties or Their Affiliates (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenues
Commissions and fees	$437,022		$275,474		$105,518
Trading revenues, net	837,357		628,304		421,063
Expenses
Execution and clearance fees	305,177		246,414		185,790
Payment for order flow	70,183		35,711		2,964
Collateralized financing interest	27,860		9,847		0
Other expense	39,814		43,094		23,073
Assets
Securities borrowed	1,632,062		1,357,387
Receivable from brokers, dealers and clearing organizations	1,188,833		1,257,251
Other assets	137,645		146,638
Liabilities
Securities loaned	707,744		733,230
Payable to brokers, dealers and clearing organizations	676,089		474,108
Related Parties [Member]
Revenues
Commissions and fees	14,528		4,045
Trading revenues, net	3,862		437
Interest, net	651		71
Total revenues from related parties	19,041		4,553
Expenses
Execution and clearance fees	-10,261	[1]	-19,491	[1]
Payment for order flow	585		0
Collateralized financing interest	529		102
Other expense	1,719		651
Total expenses incurred with respect to related parties	-7,428		-18,738
Assets
Securities borrowed	26,110		57,732
Receivable from brokers, dealers and clearing organizations	20,075		20,826
Other assets	0		277
Liabilities
Securities loaned	7,376		116,062
Payable to brokers, dealers and clearing organizations	8,509		17,911
Accrued expenses and other liabilities	$5,667		$179

[1]	Represents net volume based fees received from providing liquidity to related trading venues.

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)	0
Number of shares exercised	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	54.8
Cost of unvested awards expected to be recognized over a weighted average life, years	1 year 6 months 21 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
Stock Appreciation Rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
SARs granted (in shares)	0
Unamortized compensation costs	3.3
Award expiration period	10 months 24 days
Incentive Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred compensation payable	2.9	3.8
Accelerated amortization expense		1.3
Class B Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			3 years	3 years
Option to repurchase units after termination, period			6 months
Accelerated amortization expense		9.4
Class B Units [Member] \| Tranche 1 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of shares vesting over vesting period			33.33%
Class B Units [Member] \| Tranche 2 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of shares vesting over vesting period			33.33%
Class E Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			3 years
Award expiration period			5 years
Accelerated amortization expense		$3.50
Class E Units [Member] \| Tranche 1 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of shares vesting over vesting period			33.33%
Class E Units [Member] \| Tranche 2 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent of shares vesting over vesting period			33.33%
KCG Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grant (in shares)	30,100,000
Number of shares available for grant	16,300,000

StockBased_Compensation_Compen

Stock-Based Compensation - Compensation Expense Relating to Restricted Awards (Detail) (Restricted Stock [Member], USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock award compensation expense	$55,402	$33,067
Income tax benefit	$21,053	$11,573

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Restricted Awards Activity (Detail) (Restricted Stock Units [Member], USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014
Restricted Stock Units [Member]
Number of Units
Beginning Balance (in shares)	8,420
Granted (in shares)	4,937
Vested (in shares)	-3,464
Forfeited (in shares)	-746
Ending Balance (in shares)	9,147
Weighted- Average Grant date Fair Value
Beginning Balance (in dollars per share)	$10.71
Granted (in dollars per share)	$10.99
Vested (in dollars per share)	$10.77
Forfeited (in dollars per share)	$11.66
Ending Balance (in dollars per share)	$10.77

StockBased_Compensation_Valuat

Stock-Based Compensation - Valuation Assumptions (Details) (Stock Options [Member])	12 Months Ended
	Dec. 31, 2013
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%
Expected volatility	35.00%
Risk-free interest rate	1.00%
Expected life (in years)	3 years 6 months

StockBased_Compensation_Compen1

Stock-Based Compensation - Compensation Expense Relating to Stock Options (Detail) (Stock Options [Member], USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option and SAR compensation expense	$3,807	$1,813
Income tax benefit	$1,447	$635

StockBased_Compensation_Summar1

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014
Number of Stock Awards
Beginning Balance (in shares)	4,967,000	[1]
Granted at market value (in shares)	0
Exercised (in shares)	0
Forfeited or expired (in shares)	-276,000
Ending Balance (in shares)	4,691,000	[1]
Exercisable (in shares)	1,809,000
Available for future grants (in shares)	16,291,000	[2]
Weighted- Average Exercise Price
Beginning Balance (in dollars per share)	$18.45
Granted at market value (in dollars per share)	$0
Exercised (in dollars per share)	$0
Forfeited or expired (in dollars per share)	$36.16
Ending Balance (in dollars per share)	$17.41
Exercisable (in dollars per share)	$20.95
Aggregate Intrinsic Value of Awards, Outstanding	$3,438
Aggregate Intrinsic Value of Awards, Exercisable	$1,146
Weighted-Average Remaining Life, Outstanding	3 years 4 months 27 days
Weighted-Average Remaining Life, Exercisable	3 years 2 months 23 days
Stock Appreciation Rights (SARs) [Member]
Number of Stock Awards
Granted at market value (in shares)	1,700,000

[1]	Includes 1.7 million of SARs.
[2]	Represents options, SARs and awards available for grant

StockBased_Compensation_Exerci

Stock-Based Compensation - Exercise Price Range (Details) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	4,691
Options Outstanding, Weighted- Average Remaining Contractual Life	3 years 4 months 27 days
Options Outstanding, Weighted- Average Exercise Price (in dollars per share)	$17.41
Options Exercisable, Number Exercisable	1,809
Options Exercisable, Weighted- Average Exercise Price (in dollars per share)	$20.95
$8.24 - $8.24 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	833
Options Outstanding, Weighted- Average Remaining Contractual Life	3 years 7 months 10 days
Options Outstanding, Weighted- Average Exercise Price (in dollars per share)	$8.24
Options Exercisable, Number Exercisable	278
Options Exercisable, Weighted- Average Exercise Price (in dollars per share)	$8.24
$8.25- $8.25 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	91
Options Outstanding, Weighted- Average Remaining Contractual Life	3 years 7 months 21 days
Options Outstanding, Weighted- Average Exercise Price (in dollars per share)	$8.25
Options Exercisable, Number Exercisable	30
Options Exercisable, Weighted- Average Exercise Price (in dollars per share)	$8.25
$11.65 - $11.65 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	1,700
Options Outstanding, Weighted- Average Remaining Contractual Life	3 years 6 months 4 days
Options Outstanding, Weighted- Average Exercise Price (in dollars per share)	$11.65
Options Exercisable, Number Exercisable	567
Options Exercisable, Weighted- Average Exercise Price (in dollars per share)	$11.65
$22.50 - $22.50 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	1,700
Options Outstanding, Weighted- Average Remaining Contractual Life	3 years 6 months 4 days
Options Outstanding, Weighted- Average Exercise Price (in dollars per share)	$22.50
Options Exercisable, Number Exercisable	567
Options Exercisable, Weighted- Average Exercise Price (in dollars per share)	$22.50
$23.70 - $52.98 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	294
Options Outstanding, Weighted- Average Remaining Contractual Life	2 years 0 months 3 days
Options Outstanding, Weighted- Average Exercise Price (in dollars per share)	$40.94
Options Exercisable, Number Exercisable	294
Options Exercisable, Weighted- Average Exercise Price (in dollars per share)	$40.94
$53.91 - $53.91 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	74
Options Outstanding, Weighted- Average Remaining Contractual Life	2 years 1 month 0 days
Options Outstanding, Weighted- Average Exercise Price (in dollars per share)	$53.91
Options Exercisable, Number Exercisable	74
Options Exercisable, Weighted- Average Exercise Price (in dollars per share)	$53.91

StockBased_Compensation_Incent

Stock-Based Compensation - Incentive Units (Details) (Incentive Units [Member], Common Class A [Member])	12 Months Ended
	Dec. 31, 2014
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)	49,000
Issued (in shares)	0
Vested (in shares)	0
Exercised (in shares)	-11,000
Canceled (in shares)	0
Ending balance (in shares)	38,000

StockBased_Compensation_Compen2

Stock-Based Compensation - Compensation Expense (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	($269)	$22,306	$13,584
Class B and E Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	0	19,860	12,320
Incentive Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	($269)	$2,446	$1,264

Employee_Benefit_Plan_Addition

Employee Benefit Plan - Additional Information (Details) (Parent Company [Member])	12 Months Ended
	Dec. 31, 2014
Parent Company [Member]
Employee Benefit Plans [Line Items]
Employer contribution match to employee contributions	100.00%

Employee_Benefit_Plan_Total_Ye

Employee Benefit Plan - Total Yearly Expenses of Employee Benefit Plan (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Compensation and Retirement Disclosure [Abstract]
Total expense	$10,093	$2,959

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
Income Taxes - Additional Information (Detail) (USD $)	Jul. 01, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Contingency [Line Items]
Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	$103,500,000	$0	$103,499,000	$0
Deferred tax asset, valuation allowance, not subject to expiration			6,800,000
General business carry forward		4,400,000	2,500,000
Unrecognized tax benefits that would impact effective tax rate if recognized		2,200,000	1,500,000
Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		90,600,000	86,400,000
Deferred income tax asset		19,500,000	18,900,000
U.S. Federal [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		133,500,000	194,500,000
Deferred tax asset not subject to expiration		46,700,000	68,100,000
Deferred tax asset, valuation allowance, not subject to expiration		6,800,000
Deferred income tax asset		19,500,000	18,900,000
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		1,280,000,000	1,660,000,000
Deferred tax asset not subject to expiration		18,500,000	25,800,000
Deferred tax asset, valuation allowance, not subject to expiration		8,100,000	25,800,000
Valuation allowance reversed		10,800,000
AMT Carryforward		6,800,000	6,800,000
Knight [Member]
Income Tax Contingency [Line Items]
Deferred tax asset, net	65,465,000
Operating loss carryforwards			89,000,000
Knight [Member] \| Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		62,400,000	65,700,000
Knight [Member] \| U.S. Federal [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		$28,500,000	$89,000,000

Income_Taxes_Provision_Benefit

Income Taxes - Provision (Benefit) for Income Taxes (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
U.S. federal	$709	($194)	($627)
U.S. state and local	4,081	2,902	618
Non U.S.	-828	3,773	8,708
Current income tax expense	3,962	6,481	8,699
Deferred:
U.S. federal	30,331	-106,996	1,771
U.S. state and local	-10,997	0	1,046
Non U.S.	-543	-599	-1,240
Deferred income tax expense (benefit)	18,791	-107,595	1,577
Income tax expense (benefit)	$22,753	($101,114)	$10,276

Income_Taxes_Effective_Income_

Income Taxes - Effective Income Tax Reconciliation (Detail) (USD $)	0 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Jul. 01, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Effective Income Tax Rate Reconciliation, Deduction, Amount [Abstract]
U.S. federal statutory income tax expense		$29,815		$8,742		$9,249
Income not subject to U.S. corporate income tax		0		-15,583		-7,095
U.S. state and local income taxes, net of U.S. federal income tax effect		-4,495		1,881		772
Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	-103,500	0		-103,499		0
Nondeductible expenses		230	[1]	3,627	[1]	200	[1]
Foreign taxes		-1,371		3,603		7,468
Federal research & development tax credits		-1,241		0		0
Other, net		-185		115		-318
Income tax expense (benefit)		$22,753		($101,114)		$10,276

[1]	Nondeductible expenses include nondeductible compensation and meals and entertainment.

Income_Taxes_Components_of_Def

Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Employee compensation and benefit plans	$24,587	$16,629
Fixed assets and other amortizable assets	82,882	90,130
Reserves	2,668	5,589
Valuation of investments	32,962	30,073
Net operating loss carryforwards, net	94,770	127,637
Less: Valuation allowance on net operating loss carryforwards	-15,238	-36,043
Total deferred tax assets	222,631	234,015
Deferred tax liabilities
Employee compensation and benefit plans	0	0
Fixed assets and other amortizable assets	26,244	29,077
Reserves	2,226	0
Valuation of investments	19,912	10,414
Reduction in foreign tax credit for Non-U.S. NOL carryforwards	19,490	18,885
Total deferred tax liabilities	67,872	58,376
Net deferred tax assets	$154,759	$175,639

Income_Taxes_Unrecognized_Tax_

Income Taxes - Unrecognized Tax Benefits Rollforward (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of period	$1,464	$0
Increases based on tax positions related to prior periods	1,843	1,464
Decreases based on tax positions related to prior periods	-995	0
Decreases related to settlements with taxing authorities	0	0
Balance at the end of the period	$2,312	$1,464

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Income (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$1,401	$114,319	$0
Other comprehensive income		15,054
Amount reclassified from Accumulated Other Comprehensive Income		-127,972
Net current-period other comprehensive (loss) income	732	-112,918	114,319
Ending Balance	2,133	1,401	114,319
Non-cash gain on Knight Common Stock	0	127,972	0
Knight Capital Group [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Non-cash gain on Knight Common Stock		128,000
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	36	114,319	0
Other comprehensive income		13,689
Amount reclassified from Accumulated Other Comprehensive Income		-127,972
Net current-period other comprehensive (loss) income	316	-114,283	114,319
Ending Balance	352	36	114,319
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	1,365	0	0
Other comprehensive income		1,365
Amount reclassified from Accumulated Other Comprehensive Income		0
Net current-period other comprehensive (loss) income	416	1,365	0
Ending Balance	$1,781	$1,365	$0

Writedowns_and_Other_Charges_A

Writedowns and Other Charges - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Writedown Of Asset And Lease Loss Accrual [Line Items]
Writedown of capitalized debt costs	$9,552,000	$13,209,000	$0
Net lease loss accrual	8,625,000	14,748,000	0
Leasehold Improvements [Member]
Writedown Of Asset And Lease Loss Accrual [Line Items]
Writedown of assets		7,700,000
Excess Real Estate Capacity [Member]
Writedown Of Asset And Lease Loss Accrual [Line Items]
Net lease loss accrual	8,600,000	7,100,000
Secured Debt [Member]
Writedown Of Asset And Lease Loss Accrual [Line Items]
Early principal payments	235,000,000	300,000,000
Writedown of capitalized debt costs	$9,600,000	$13,200,000

Earnings_Per_Share_Additional_

Earnings Per Share - Additional Information (Detail)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Options excluded (in shares)	28.3	29.1

Earnings_Per_Share_Reconciliat

Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted (Loss) Earnings Per Computations (Detail) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator / net income
Income from continuing operations and shares used in basic calculations	$62,434	$126,092	$16,151
Income from continuing operations and shares used in diluted calculations	62,434	126,092	16,151
Loss from continuing operations allocated to preferred and participating units	0	-21,565	1,092
Income from continuing operations attributable to common shareholders	$62,434	$147,657	$15,059
Denominator / shares
Income from continuing operations and shares used in basic calculations (in shares)	112,854	80,143	48,970
Income from continuing operations and shares used in diluted calculations (in shares)	116,534	81,015	48,970
Basic earnings per share from continuing operations (in dollars per share)	$0.55	$1.84	$0.31
Diluted earnings per common share from continuing operations (in dollars per share)	$0.54	$1.82	$0.31
Restricted Awards [Member]
Denominator / shares
Effect of dilutive stock based awards (in shares)	3,579	855	0
Stock Options and SARs [Member]
Denominator / shares
Effect of dilutive stock based awards (in shares)	101	17	0

Significant_Clients_Additional

Significant Clients - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Client	Client	Client
Risks and Uncertainties [Abstract]
Number of significant clients	0	0	0
Percentage of dollar value traded	10.00%	10.00%	10.00%

Recovered_Sheet1

Commitments and Contingent liabilities - Additional Information (Detail) (USD $)	12 Months Ended		0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Aug. 06, 2012	Aug. 01, 2012	Sep. 26, 2014	Jan. 15, 2013	Jun. 05, 2014
	Dec. 31, 2014	Dec. 31, 2013	Aug. 06, 2012	lawsuit	Sep. 26, 2014	director	Jun. 05, 2014
Loss Contingencies [Line Items]
Rental expense	$19,700,000	$16,000,000
Letters of credit held in escrow	11,600,000
Compensation guarantees payable	9,200,000	0
Revolving Credit Facility [Member]
Loss Contingencies [Line Items]
Weighted average interest rate	3.52%
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
New York Litigation [Member]
Loss Contingencies [Line Items]
Number of directors accused of breaking fiduciary duties						7
Pre-tax loss settlements realized						457,600,000
Attorneys' fees and costs					425,000
New York Litigation [Member] \| Maximum [Member]
Loss Contingencies [Line Items]
Attorneys' fees and costs							$490,000

Recovered_Sheet2

Commitments and Contingent liabilities - Schedule of Capital Lease and Contract Obligations (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified	Dec. 31, 2014
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015	$4,223
2016	2,126
2017	620
Total	$6,969

Commitments_and_Contingent_lia2

Commitments and Contingent liabilities - Schedule of Interest Expense (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Interest expense - Capital leases	$370	$700	$1,376

Commitments_and_Contingent_lia3

Commitments and Contingent liabilities - Schedule of Operating Lease and Contract Obligations (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified	Dec. 31, 2014
Gross Lease Obligations [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Year ending December 31, 2015	$28,224
Year ending December 31, 2016	27,762
Year ending December 31, 2017	26,211
Year ending December 31, 2018	25,354
Year ending December 31, 2019	23,316
Thereafter through December 31, 2027	64,441
Total	195,308
Sublease Income [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Year ending December 31, 2015	4,056
Year ending December 31, 2016	3,488
Year ending December 31, 2017	2,983
Year ending December 31, 2018	2,667
Year ending December 31, 2019	2,389
Thereafter through December 31, 2027	10,244
Total	25,827
Net Lease Obligations [Member]
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Year ending December 31, 2015	24,168
Year ending December 31, 2016	24,274
Year ending December 31, 2017	23,228
Year ending December 31, 2018	22,687
Year ending December 31, 2019	20,927
Thereafter through December 31, 2027	54,197
Total	$169,481

Net_Capital_Requirements_Net_C

Net Capital Requirements - Net Capital Levels and Requirements for U.S. Registered Broker-Dealer Subsidiary (Details) (KCG Americas LLC [Member], USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified	Dec. 31, 2014
KCG Americas LLC [Member]
Net Capital And Net Capital Requirements For Companys Broker Dealer Subsidiaries [Line Items]
Net Capital	$285,231
Net Capital Requirement	1,000
Excess Financial Resources	$284,231

Net_Capital_Requirements_Finan

Net Capital Requirements - Financial Resource Requirements on Significant Foreign Regulated Broker-Dealer (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Net Capital Requirements [Line Items]
Release of excess capital	$45,000,000
KCG Europe Limited [Member]
Net Capital Requirements [Line Items]
Net Capital	187,940,000
Resource Requirement	-124,577,000
Excess Financial Resources	63,363,000
GETCO Europe Limited [Member]
Net Capital Requirements [Line Items]
Net Capital	7,254,000
Resource Requirement	-1,759,000
Excess Financial Resources	$5,495,000

Financial_instruments_with_Off1

Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Loans outstanding	$422,259,000	$657,259,000
Executive Director or Officer [Member]
Derivatives, Fair Value [Line Items]
Loans outstanding	$0

Business_Segments_Detail

Business Segments (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues	$1,316,232	$1,027,336	$551,234
Pre-tax earnings	85,187	24,978	26,427
Total assets	6,830,654	6,997,004	1,687,536
U.S. [Member]
Segment Reporting Information [Line Items]
Revenues	1,127,088	834,410	340,117
International [Member]
Segment Reporting Information [Line Items]
Revenues	189,144	192,926	211,117
Market Making [Member]
Segment Reporting Information [Line Items]
Revenues	901,152	688,197	495,427
Pre-tax earnings	146,713	103,612	34,887
Total assets	4,401,021	3,939,965	1,408,516
Global Execution Services [Member]
Segment Reporting Information [Line Items]
Revenues	345,710	197,765	36,211
Pre-tax earnings	11,056	-25,794	-7,330
Total assets	786,734	1,106,448	33,623
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	69,369	141,374	19,596
Pre-tax earnings	-72,582	-52,842	-1,130
Total assets	$1,642,899	$1,950,591	$245,397

Business_Segments_Additional_I

Business Segments - Additional Information (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Debt interest expense	$32,456	$34,938	$2,665
FCM [Member]
Segment Reporting Information [Line Items]
Total assets held for sale	40,484
Market Making [Member]
Segment Reporting Information [Line Items]
Debt interest expense	24,700
Global Execution Services [Member]
Segment Reporting Information [Line Items]
Debt interest expense	7,100
Global Execution Services [Member] \| FCM [Member]
Segment Reporting Information [Line Items]
Total assets held for sale	$40,500

Condensed_Financial_Statements2

Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Financial Condition (Parent Only) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$578,768,000	$674,281,000		$427,631,000	$607,689,000
Other assets	137,645,000	146,638,000
Total assets	6,830,654,000	6,997,004,000		1,687,536,000
Liabilities
Accrued compensation expense	114,559,000	149,430,000
Accrued expenses and other liabilities	136,977,000	175,910,000
Debt	422,259,000	657,259,000
Total liabilities	5,308,077,000	5,487,467,000
Total equity	1,522,577,000	1,509,537,000	613,470,000	654,672,000	622,996,000
Total liabilities and equity	6,830,654,000	6,997,004,000
Parent Company [Member]
Assets
Cash and cash equivalents	302,700,000	253,017,000		114,511,000	112,496,000
Receivable from subsidiaries	21,851,000	60,460,000
Investments in subsidiaries	1,122,254,000	1,359,779,000
Income taxes receivable	13,999,000	90,000
Deferred tax asset, net	137,009,000	143,028,000
Subordinated loans to subsidiaries	280,000,000	250,000,000
Other assets	19,397,000	21,182,000
Total assets	1,897,210,000	2,087,556,000
Liabilities
Accrued compensation expense	13,208,000	0
Payable to subsidiaries	42,478,000	25,705,000
Accrued expenses and other liabilities	13,947,000	12,314,000
Debt	305,000,000	540,000,000
Total liabilities	374,633,000	578,019,000
Total equity	1,522,577,000	1,509,537,000
Total liabilities and equity	$1,897,210,000	$2,087,556,000

Condensed_Financial_Statements3

Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Operations (Parent Only) (Details) (USD $)	6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Investment income and other, net			$41,232	$124,095	$27,010
Total revenues			1,316,232	1,027,336	551,234
Expenses
Employee compensation and benefits			437,269	349,192	157,855
Depreciation and amortization			81,448	55,570	34,938
Professional fees			25,596	46,662	14,072
Business development			9,763	4,609	23
Other			39,814	43,094	23,073
Total expenses			1,231,045	1,002,358	524,807
Income from continuing operations before income taxes			85,187	24,978	26,427
Income tax benefit			22,753	-101,114	10,276
Income from continuing operations, net of tax			62,434	126,092	16,151
Net income	209,393	-89,305	61,102	126,172	16,151
Parent Company [Member]
Revenues
Investment income and other, net			3,415	3,717	5,981
Total revenues			3,415	3,717	5,981
Expenses
Employee compensation and benefits			50,256	31,970	-12,251
Interest			11,939	28,476	1,195
Depreciation and amortization			0	698	1,395
Professional fees			9,211	30,488	4,997
Business development			3,625	0	0
Other			25,460	29,829	1,493
Total expenses			100,491	121,461	-3,171
Income from continuing operations before income taxes			-97,076	-117,744	9,152
Income tax benefit			-35,972	-120,761	0
Income from continuing operations, net of tax			-61,104	3,017	9,152
Equity in earnings of subsidiaries			122,206	123,155	6,999
Net income			$61,102	$126,172	$16,151

Condensed_Financial_Statements4

Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Statements of Cash Flows (Parent Only) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities
Net income	$61,102,000	$126,172,000	$16,151,000
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by operating activities
Writedown and amortization of debt issuance costs	13,217,000	17,332,000	276,000
Amortization expense	35,592,000	19,211,000	5,518,000
Stock-based compensation	58,940,000	64,286,000	12,320,000
(Increase) decrease in operating assets
Subordinated loan receivable	14,468,000	-129,035,000	-414,537,000
Deferred tax asset	19,397,000	-103,499,000	0
Other assets	-6,459,000	25,785,000	1,332,000
(Decrease) increase in operating liabilities
Accrued compensation expense	-29,536,000	24,441,000	-18,494,000
Accrued expenses and other liabilities	-40,583,000	-64,470,000	-13,779,000
Net cash provided by (used in) operating activities	242,524,000	141,204,000	-64,516,000
Cash flows from investing activities
Net cash provided by investing activities	11,472,000	582,892,000	-46,501,000
Cash flows from financing activities
Partial repayment of Credit Agreement	-235,000,000	-300,000,000	0
Repayment of notes payable	0	-15,000,000	0
Payment of debt issuance costs	0	-34,592,000	-1,375,000
Issuance of equity to General Atlantic	0	55,000,000	0
Payment to former Knight Capital Group, Inc. stockholders	0	-720,000,000	0
Repayment of Knight Convertible Notes		-257,700,000
Funding of collateral account for Knight Convertible Notes	0	-117,259,000	0
Payment out of collateral account for Knight Convertible Notes	0	117,259,000	0
Members' distributions	0	-21,002,000	-37,061,000
Cost of common stock repurchased	-111,585,000	-11,324,000	0
Net cash used in financing activities	-349,925,000	-478,811,000	-69,041,000
Effect of exchange rate changes on cash and cash equivalents	416,000	1,365,000	0
Increase in cash and cash equivalents	-95,513,000	246,650,000	-180,058,000
Cash and cash equivalents at beginning of period	674,281,000	427,631,000	607,689,000
Cash and cash equivalents at end of period	578,768,000	674,281,000	427,631,000
Supplemental disclosure of cash flow information:
Cash paid for interest	76,003,000	56,227,000	8,084,000
Cash paid for income taxes	16,975,000	10,198,000	21,789,000
Parent Company [Member]
Cash flows from operating activities
Net income	61,102,000	126,172,000	16,151,000
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by operating activities
Equity in earnings of subsidiaries	-122,206,000	-123,155,000	-6,999,000
Writedown and amortization of debt issuance costs	12,548,000	16,931,000	52,000
Amortization expense	0	698,000	1,395,000
Stock-based compensation	16,997,000	0	0
(Increase) decrease in operating assets
Subordinated loan receivable	-30,000,000	-250,000,000	0
Receivable from subsidiaries	38,609,000	-67,523,000	-97,375,000
Deferred tax asset	-13,909,000	-144,109,000	0
Income taxes receivable	6,019,000	-90,000	0
Other assets	-10,763,000	-15,351,000	31,031,000
(Decrease) increase in operating liabilities
Payable to subsidiaries	16,773,000	-2,024,000	11,756,000
Accrued compensation expense	8,839,000	0	0
Accrued expenses and other liabilities	1,633,000	59,561,000	5,106,000
Net cash provided by (used in) operating activities	-14,358,000	-398,890,000	-38,883,000
Cash flows from investing activities
Cash acquired upon acquisition of Knight Capital Group, Inc.	0	509,133,000	0
Cash received from sale of Urban Financial of America, LLC	0	85,406,000	0
Dividends received from subsidiaries	807,715,000	646,425,000	161,606,000
Capital contributions to subsidiaries	-397,089,000	-238,909,000	-70,794,000
Net cash provided by investing activities	410,626,000	1,002,055,000	90,812,000
Cash flows from financing activities
Proceeds from issuance of Credit Agreement	0	535,000,000	0
Partial repayment of Credit Agreement	-235,000,000	-300,000,000	0
Proceeds from issuance of Senior Secured Notes	0	305,000,000	0
Repayment of notes payable	0	-15,000,000	0
Payment of debt issuance costs	0	-34,592,000	0
Issuance of equity to General Atlantic	0	55,000,000	0
Payment to former Knight Capital Group, Inc. stockholders	0	-720,000,000	0
Repayment of Knight Convertible Notes	0	-257,741,000	0
Funding of collateral account for Knight Convertible Notes	0	-117,259,000	0
Payment out of collateral account for Knight Convertible Notes	0	117,259,000	0
Proceeds from stock issuance	0	0	12,320,000
Members' distributions	0	-21,002,000	-62,234,000
Cost of common stock repurchased	-111,585,000	-11,324,000	0
Net cash used in financing activities	-346,585,000	-464,659,000	-49,914,000
Effect of exchange rate changes on cash and cash equivalents	0	0	0
Increase in cash and cash equivalents	49,683,000	138,506,000	2,015,000
Cash and cash equivalents at beginning of period	253,017,000	114,511,000	112,496,000
Cash and cash equivalents at end of period	302,700,000	253,017,000	114,511,000
Supplemental disclosure of cash flow information:
Cash paid for interest	28,426,000	26,239,000	1,116,000
Cash paid for income taxes	$15,456,000	$365,000	$0

Subsequent_Event_Details

Subsequent Event (Details) (Subsequent Event [Member], KCG Hotspot [Member], USD $)	1 Months Ended
In Millions, unless otherwise specified	Jan. 31, 2015
Subsequent Event [Line Items]
Cash to be received upon close	$365
Additional proceeds related to tax benefits	70
Sale period	3 years
Percentage share in actual tax benefits for the first three years after closing	70.00%
Percentage share in actual tax benefits thereafter	50.00%
Period to exercise one-time option to terminate the continued tax sharing arrangement	30 days
Payment to be received in exchange of the option to terminate the continued tax sharing arrangement	$50
Period without entitlement to annual tax benefits	3 years
Trigger Event [Member]
Subsequent Event [Line Items]
Rolling period for the average daily notional FX trading volume	60 days
Average daily notional foreign exchange trading volume, threshold percentage	70.00%
After Trigger Event [Member]
Subsequent Event [Line Items]
Rolling period for the average daily notional FX trading volume	15 days
Average daily notional foreign exchange trading volume, threshold percentage	80.00%