KCG 10-Q 12 Nov 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Nov. 08, 2013	Nov. 08, 2013
		Common Class A [Member]	Common Class B [Member]
Entity Information [Line Items]	'	'	'
Entity Registrant Name	'KCG HOLDINGS, INC.	'	'
Entity Central Index Key	'0001569391	'	'
Document Type	'10-Q	'	'
Document Period End Date	30-Sep-13	'	'
Amendment Flag	'false	'	'
Document Fiscal Year Focus	'2013	'	'
Document Fiscal Period Focus	'Q3	'	'
Current Fiscal Year End Date	'--12-31	'	'
Entity Filer Category	'Large Accelerated Filer	'	'
Entity Common Stock, Shares Outstanding	'	122,430,198	0

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues	'	'	'	'
Trading revenues, net	$230,471	$92,556	$415,495	$333,737
Commissions and fees	109,079	25,542	164,391	79,487
Interest, net	-177	-781	-970	-1,998
Investment income and other, net	128,446	13,285	119,207	14,015
Total revenues	467,819	130,602	698,123	425,241
Expenses	'	'	'	'
Employee compensation and benefits	129,631	31,875	236,983	111,395
Execution and clearance fees	81,023	42,267	167,931	144,656
Communications and data processing	44,046	21,681	86,040	67,380
Interest	23,870	706	26,515	2,002
Depreciation and amortization	20,091	7,574	36,004	27,180
Payments for order flow	16,431	717	17,468	2,128
Professional fees	9,077	2,575	38,928	7,690
Occupancy and equipment rentals	8,898	3,240	15,454	8,865
Business development	2,644	6	2,686	19
Writedown of assets and lease loss accrual	936	0	4,248	0
Other	11,318	5,349	30,028	19,002
Total expenses	347,965	115,990	662,285	390,317
Income from continuing operations before income taxes	119,854	14,612	35,838	34,924
Income tax (benefit) expense	-107,767	4,805	-102,478	10,368
Income from continuing operations, net of tax	227,621	9,807	138,316	24,556
Loss from discontinued operations, net of tax	-784	0	-784	0
Net income	$226,837	$9,807	$137,532	$24,556
Basic earnings per share from continuing operations (in dollars per share)	$1.99	$0.21	$2.02	$0.49
Diluted earnings per share from continuing operations (in dollars per share)	$1.98	$0.21	$2.01	$0.49
Basic loss per share from discontinued operations (in dollars per share)	($0.01)	$0	($0.01)	$0
Diluted loss per share from discontinued operations (in dollars per share)	($0.01)	$0	($0.01)	$0
Basic earnings per share (in dollars per share)	$1.99	$0.21	$2	$0.49
Diluted earnings per share (in dollars per share)	$1.98	$0.21	$2	$0.49
Shares used in computation of basic earnings (loss) per share (in shares)	114,113	46,411	68,632	49,619
Shares used in computation of diluted earnings (loss) per share (in shares)	114,773	46,411	68,855	49,619

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net income	$226,837	$9,807	$137,532	$24,556
Unrealized (loss) gain on available for sale securities, net of tax	-118,918	66,704	-114,368	66,704
Cumulative translation adjustment, net of tax	-4,695	0	-5,098	0
Comprehensive income	$103,224	$76,511	$18,066	$91,260

CONSOLIDATED_STATEMENTS_OF_FIN

CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (USD $)	Sep. 30, 2013	Dec. 31, 2012
Assets	'	'
Cash and cash equivalents	$798,712,000	$427,631,000
Cash and cash equivalents segregated under federal and other regulations	216,442,000	0
Financial instruments owned, at fair value, including securities pledged to counterparties that had the right to deliver or repledge of $961,479 at September 30, 2013 and $52,261 at December 31, 2012:	'	'
Equities	2,156,466,000	378,933,000
Listed options	288,227,000	92,305,000
Debt securities	79,284,000	183,637,000
Total financial instruments owned, at fair value	2,523,977,000	654,875,000
Collateralized agreements:	'	'
Securities borrowed	1,370,921,000	52,261,000
Receivable from brokers, dealers and clearing organizations	1,330,113,000	142,969,000
Fixed assets and leasehold improvements, less accumulated depreciation and amortization	161,865,000	83,341,000
Investments	125,889,000	248,438,000
Goodwill	18,398,000	4,645,000
Intangible assets, less accumulated amortization	192,045,000	46,123,000
Deferred tax asset	169,619,000	4,180,000
Assets within discontinued operations	6,098,299,000	0
Other assets	287,015,000	23,073,000
Total assets	13,293,295,000	1,687,536,000
Financial instruments sold, not yet purchased, at fair value:	'	'
Equities	1,848,728,000	423,740,000
Listed options	229,722,000	69,757,000
Debt securities	79,057,000	19,056,000
Other financial instruments	5,431,000	0
Total financial instruments sold, not yet purchased, at fair value	2,162,938,000	512,553,000
Collateralized financings:	'	'
Securities loaned	543,451,000	0
Financial instruments sold under agreements to repurchase	595,029,000	0
Total collateralized financings	1,138,480,000	0
Payable to brokers, dealers and clearing organizations	666,178,000	24,185,000
Payable to customers	486,136,000	0
Accrued compensation expense	130,158,000	27,728,000
Accrued expenses and other liabilities	220,648,000	118,068,000
Capital lease obligations	12,453,000	24,191,000
Liabilities within discontinued operations	6,006,024,000	0
Short-term debt	235,000,000	0
Long-term debt	722,259,000	15,000,000
Total liabilities	11,780,274,000	721,725,000
Redeemable preferred member's equity	0	311,139,000
Members' equity	0	654,672,000
Equity	'	'
Additional paid-in capital	1,299,907,000	0
Retained earnings	226,837,000	0
Treasury stock, at cost; 928 shares at September 30, 2013	-9,811,000	0
Accumulated other comprehensive loss	-5,147,000	0
Total equity	1,513,021,000	654,672,000
Total liabilities, redeemable preferred member's equity and equity	13,293,295,000	1,687,536,000
Common Class A [Member]	'	'
Equity	'	'
Shares authorized: 1,000,000 at September 30, 2013; Shares issued: 123,515 at September 30, 2013; Shares outstanding: 122,587 at September 30, 2013	$1,235,000	$0

CONSOLIDATED_STATEMENTS_OF_FIN1

CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, except Share data, unless otherwise specified
Financial instruments owned, at fair value, securities pledged	$961,479	$55,141
Treasury stock, shares	928,000	0
Common Class A [Member]	'	'
Class A common stock Shares authorized	1,000,000,000	0
Class A common stock, Shares issued	123,515,000	0
Class A common stock, Shares outstanding	122,587,000	0

CONSOLIDATED_STATEMENTS_OF_CHA

CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $)	Total	Redeemable preferred members' 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 loss [Member]	Knight Capital Group, Inc., Total [Member]
In Thousands, unless otherwise specified
Balance at Dec. 31, 2012	$654,672	$311,139	$559,292	($18,939)	$0	$0	$0	$0	$114,319	$654,672
Balance, Shares at Dec. 31, 2012	'	'	'	'	0	'	'	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 loss	'	'	-89,305	'	'	'	'	'	4,147	-85,158
Modification of redemption value	'	-21,565	21,565	'	'	'	'	'	'	21,565
Balance at Jun. 30, 2013	'	289,574	495,004	0	0	0	0	0	118,466	613,470
Balance, Shares at Jun. 30, 2013	'	'	'	'	0	'	'	0	'	'
Increase (Decrease) in Stockholders' Equity [Roll Forward]	'	'	'	'	'	'	'	'	'	'
Net loss	226,837	'	'	'	'	'	226,837	'	'	226,837
Gain on investment in Knight Capital Group, Inc.	'	'	'	'	'	'	'	'	9,103	9,103
Equity issued to General Atlantic	55,000	'	55,000	'	'	'	'	'	'	'
Exchange of membership interest for shares of KCG Class A Common Stock (in shares)	'	'	'	'	75,868	'	'	'	'	'
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 stockholders	'	'	'	'	419	459,190	'	'	'	459,609
Issuance of KCG Class A Common Stock, Shares	'	'	'	'	41,889	'	'	'	'	'
KCG Common Stock repurchased	'	'	'	'	'	'	'	-9,811	'	-9,811
Knight common stock repurchased, Shares	'	'	'	'	'	'	'	-928	'	'
Stock-based compensation	'	'	'	'	57	11,404	'	'	'	11,461
Stock-based compensation, Shares	'	'	'	'	5,758	'	'	'	'	'
Unrealized (loss) gain on available for sale securities, net of tax	'	'	'	'	'	'	'	'	-49	-49
Cumulative translation adjustment	'	'	'	'	'	'	'	'	-4,695	-4,695
Reclassification of investment in Knight out of other comprehensive income	'	'	'	'	'	'	'	'	-127,972	-127,972
Balance at Sep. 30, 2013	$1,513,021	$0	$0	$0	$1,235	$1,299,907	$226,837	($9,811)	($5,147)	$1,513,021
Balance, Shares at Sep. 30, 2013	'	'	'	'	123,515	'	'	928	'	'

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities	'	'
Net income	$137,532	$24,556
Loss from discontinued operations, net of tax	784	0
Income from continuing operations, net of tax	138,316	24,556
Adjustments to reconcile income from continuing operations, net of tax to net cash provided by (used in) operating activities	'	'
Non-cash gain on Knight Common Stock	-127,972	0
Deferred tax benefit	-103,499	0
Depreciation and amortization	36,004	27,180
Stock and unit-based compensation	41,537	10,152
Unrealized gain (loss) on investments	5,625	-12,705
Writedown of assets and lease loss accrual	4,248	0
Amortization of debt offering costs	2,479	427
Deferred rent	-247	1,422
Operating activities from discontinued operations	-962	0
(Increase) decrease in operating assets	'	'
Cash and securities segregated under federal and other regulations	-13,397	0
Financial instruments owned, at fair value	68,827	-503,396
Securities borrowed	-159,679	-32,673
Receivable from brokers, dealers and clearing organizations	179,403	33,235
Other assets	31,059	3,837
Financial instruments sold, not yet purchased, at fair value	'	'
Financial instruments sold, not yet purchased, at fair value	137,389	344,087
Securities loaned	-83,440	0
Financial instruments sold under agreements to repurchase	50,029	0
Payable to brokers, dealers and clearing organizations	6,079	79,226
Payable to customers	-41,782	0
Accrued compensation expense	12,537	-34,291
Accrued expenses and other liabilities	-21,988	-22,184
Net cash provided by (used in) operating activities	160,566	-81,127
Cash flows from investing activities	'	'
Cash acquired upon acquisition of Knight Capital Group, Inc.	509,133	0
Proceeds and distributions from investments	3,251	19,820
Purchases of investments	-17,506	-35,631
Purchases of investments	-85	-89,689
Investing activities from discontinued operations	-12,963	0
Net Cash Provided by (Used in) Investing Activities	507,756	-105,500
Cash flows from financing activities	'	'
Proceeds from issuance of Credit Agreement	535,000	0
Proceeds from Senior Secured Notes	305,000	0
Payment of debt issuance cost	-34,592	0
Issuance of equity to General Atlantic	55,000	0
Payment to former Knight Capital Group, Inc. stockholders	-720,000	0
Repayment of Knight Convertible Notes	-257,741	0
Funding of collateral account for Knight Convertible Notes	-117,259	0
Borrowings under unsecured credit facility	0	46,750
Borrowings under secured credit facility	25,000	0
Repayment of secured credit facility	-25,000	0
Repayment of notes	-15,000	0
Borrowings under capital lease obligations	0	13,758
Principal payments on capital lease obligations	-11,738	-10,679
Members' distributions	-21,002	-47,383
Cost of common stock repurchased	-9,811	0
Net cash used in financing activities	-292,143	2,446
Effect of exchange rate changes on cash and cash equivalents	-5,098	0
Increase (decrease) in cash and cash equivalents	371,081	-184,181
Cash and cash equivalents at beginning of period	427,631	607,689
Cash and cash equivalents at end of period	798,712	423,508
Supplemental disclosure of cash flow information:	'	'
Cash paid for interest	25,734	6,097
Cash paid for income taxes	$9,071	$19,932

Organization_and_Description_o

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

Merger_of_GETCO_and_Knight

Merger of GETCO and Knight	9 Months Ended
	Sep. 30, 2013
Business Combinations [Abstract]	'
Merger With GETCO and Knight	'
	Merger of GETCO and Knight
	Background
	Pursuant to the Merger Agreement, upon completion of the Mergers, subject to proration and certain specified exceptions, 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”). Pursuant to the proration procedures provided in the Merger Agreement and taking into account the waiver by Jefferies LLC, Knight's largest stockholder before the Mergers, of its right to receive cash consideration with respect to certain of its shares, former Knight stockholders eligible for election received a cash payment of approximately $720.0 million.
	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”).
	Upon completion of the Mergers, Knight's stockholders received, in the aggregate, $720.0 million in cash and 41.9 million shares of KCG Class A Common Stock. After taking into account the election results and the shares of KCG Class A Common Stock issued to former unitholders of GETCO and former stockholders of Knight, 116.8 million shares (including unvested restricted stock units ("RSUs")) of KCG Class A Common Stock were outstanding as of July 1, 2013.
	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 nine months ended September 30, 2013 comprise third quarter results of KCG and the results of GETCO for the six months ended June 30, 2013. All periods prior to 2013 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 within equity. On the acquisition date, as a result of the Mergers, the Company reversed the cumulative gain that it had recorded in other comprehensive income within equity and recognized a gain of $128.0 million in Investment income and other, net on its Consolidated Statements of Operations for the three and nine months ended September 30, 2013.
	All GETCO earnings per share and unit share outstanding amounts in this Quarterly Report on Form 10-Q have been calculated as if the conversion of GETCO units to KCG Class A Common Stock took place on January 1, 2012, at the exchange ratio, as defined in the Merger Agreement. See Footnote 15 "Earnings Per Share".
	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. The Company has not yet completed all of its analyses to finalize the allocation of the purchase price to the Knight acquired assets and liabilities. The allocation of the purchase price may be modified over the measurement period, as more information is obtained about the fair values of assets acquired and liabilities assumed.
	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 and measures them using the enacted tax rates and laws that will be in effect when such differences are expected to reverse.
	The Company recorded net deferred tax assets of $65.9 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 losses ('NOLs”) and other tax attributes acquired as a result of the Mergers, as described in Footnote 13 “Income Taxes”. These deferred tax assets are included in Other assets in the table below.
	The following table reflects the preliminary 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 securities 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				1,366,974
	Fixed assets and leasehold improvements, net				84,596
	Investments				106,353
	Intangible assets				156,800
	Assets within discontinued operations				5,607,063
	Other assets				211,735
	Total Assets				$	11,342,609
	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				139,624
	Liabilities within discontinued operations				5,518,168
	Long-term debt				375,000
	Total Liabilities				$	9,983,227
	Total identified assets acquired, net of assumed liabilities				1,359,382
	Goodwill				13,753
	Total Purchase Price				$	1,373,135
	Amounts preliminarily allocated to intangible assets, the amortization period and goodwill were as follows (dollars in thousands):
							Amortization
			Amount				Years
	Technology		$	110,000			5 years
	Customer relationships		35,000				9 - 11 years
	Trade names		4,000				10 years
	Trading rights		7,800				7 years
	Intangible assets		156,800
	Goodwill		13,753
	Total		$	170,553
	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.
	The Company recorded $1.2 million and $27.6 million of costs related to the Mergers for the three and nine months ended September 30, 2013, respectively.
	Included in KCG results for the three months ended September 30, 2013 are results from the businesses acquired as a result of the Mergers as follows:
		Three months ended September 30,
		2013
	Revenues	$	240,805
	Income from continuing operations, before income taxes	21,164
			Three months ended September 30, 2012							Nine months ended September 30, 2013					Nine months ended September 30, 2012
			Reported				Pro Forma			Reported		Pro Forma			Reported		Pro Forma
			(in thousands, except per share amounts)
	Revenue		$	130,602			$	(120,210	)	$	698,123	$	1,099,518		$	425,241	$	677,955
	Net income from continuing operations		9,807				(303,413		)	138,316		(10,494		)	24,556		(274,981		)
	Net income		9,807				(387,350		)	137,532		(39,554		)	24,556		(350,831		)
Diluted earnings (loss) per share from continuing operations		$	0.21			$	(6.54	)	$	2.01	$	(0.15	)	$	0.49	$	(5.54	)
Diluted earnings (loss) per share		$	0.21			$	(8.35	)	$	2	$	(0.57	)	$	0.49	$	(7.07	)
The pro forma results are based on adding the pre-tax historical results of GETCO and Knight, and adjusting primarily for amortization of intangibles created in the Mergers, debt raised in conjunction with the Mergers and income taxes as if the Company was subject to U.S. corporate income taxes for all periods presented. The pro forma data assumes all GETCO units have been converted to KCG Class A Common Stock on January 1, 2012 and excludes any gain recognized on Knight Common Stock. The pro forma disclosures do not include adjustments to reflect the Company's operating costs or expected differences in the way funds generated by the Company are invested. The pro forma data is intended for informational purposes and is not indicative of the future results of operations.

Significant_Accounting_Policie

Significant Accounting Policies

9 Months Ended

Sep. 30, 2013

Accounting Policies [Abstract]

'

Significant Accounting Policies

'

Significant Accounting Policies

Basis of consolidation and form of presentation

The accompanying unaudited Consolidated Financial Statements, prepared in conformity with GAAP, include the accounts of the Company and its subsidiaries and should be read in conjunction with the audited consolidated financial statements of GETCO for the year ended December 31, 2012 contained in the Current Report on Form 8-K filed by the Company on November 12, 2013 and in the Current Report on Form 8-K filed by the Company on July 1, 2013 and with the audited consolidated financial statements of Knight for the year ended December 31, 2012 in the Current Report on Form 8-K filed by Knight on May 13, 2013. 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.

The Company consolidates all of its subsidiaries as well as any variable interest entity (“VIE”) for which it is considered to be the primary beneficiary. The Company performs a qualitative assessment to determine if a VIE should be consolidated. As described in more detail below, the primary attributes the Company assesses include the entity’s capital structure and power. The Company will consolidate a VIE if it has both i) the power to direct the activities of a VIE that most significantly impact the entity’s economic performance and ii) the obligation to absorb losses of the entity that could potentially be significant to the VIE or the right to receive benefits from the entity that could potentially be significant to the VIE. As of September 30, 2013 and December 31, 2012, the Company was not considered to be a primary beneficiary of any VIE.

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.

Cash and securities segregated under federal and other regulations

The Company maintains custody of customer funds and, as a result, it is subject to various regulatory rules and regulations. As a result of these customer holdings, the Company is obligated by the U.S. Securities and Exchange Commission (“SEC”) and the Commodities 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 securities 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, which relate to market making and trading activities, include listed and OTC equity securities, listed equity options and fixed income securities which are recorded on a trade date basis and carried at fair value. Trading revenue, net (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 FCM are recorded net of any commissions paid to independent brokers and are 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 clearing brokers, for facilitating the settlement and financing of securities transactions. Interest income and interest expense which have been netted within Interest, net on the Consolidated Statements of Operations are as follows (in thousands):

For the three months

For the nine months

ended September 30,

ended September 30,

2013

2012

2013

2012

Interest Income

$

3,992

$

1,170

$

6,022

$

1,847

Interest Expense

(4,169

)

(1,951

)

(6,992

)

(3,845

)

Interest, net

$

(177

)

$

(781

)

$

(970

)

$

(1,998

)

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

For the three months

For the nine months

ended September 30,

ended September 30,

2013

2012

2013

2012

Dividend Income

$

9,009

$

288

$

11,484

$

1,049

Dividend Expense

$

(6,899

)

$

(809

)

$

(8,035

)

$

(1,321

)

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, with additional collateral obtained or refunded 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.

Collateralized agreements related to Urban are recorded within Liabilities within discontinued operations on the Consolidated Statements of Financial Condition, and comprise:

•

Other secured financings, which represent contractual agreements used to finance financial instruments, are recorded at their contractual amount. These agreements are short-term in nature with durations of typically less than one month and bear interest at negotiated rates. Urban receives cash and pledges financial instruments to banks and other financial institutions as collateral for these secured financing arrangements. The market value of the collateral delivered must be in excess of the principal amount loaned plus the agreed upon margin requirement under the secured financings. The banks and other financial institutions may request additional collateral, if appropriate.

•

Liability to GNMA trusts, at fair value, represent the liability associated with Urban’s securitization of HECMs where the securitization does not meet the GAAP criteria for sale treatment.

The Company’s securities borrowed, securities loaned, financial instruments sold under agreements to repurchase and other secured financings 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 fully collateralized. These items would be categorized as Level 2 in the fair value hierarchy if they were required to be recorded at fair value.

Securitization activities

Urban, which KCG has entered into a definitive agreement to sell as described in Footnote 1 "Organization and Description of the Business" and Footnote 4 "Discontinued Operations", securitizes home equity conversion mortgages (“HECMs”) under its GNMA issuance authority. Securitization and transfer of financial assets to a third party are generally accounted for as sales when an issuer has relinquished control over the transferred assets. Based upon the current structure of the GNMA securitization program, the Company believes that Urban has not met the GAAP criteria for relinquishing control over the transferred assets and therefore its securitizations fail to meet the GAAP criteria for sale accounting. As such, the Company continues to recognize the HECMs in Assets within discontinued operations, and the Company recognizes a corresponding liability to the GNMA trusts in Liabilities within discontinued operations on the Consolidated Statements of Financial Condition. The associated change in fair value of the securitized HECM loan inventory is recorded in Loss from discontinued operations, net of tax on the Consolidated Statements of Operations.

Investments

Investments primarily comprise strategic investments and deferred compensation investments. Strategic investments include noncontrolling equity ownership interests held by the Company within its non-broker-dealer subsidiaries, primarily in financial services-related businesses. Strategic investments are accounted for under the equity method or at cost. The equity method of accounting is used when the Company has significant influence, generally considered to be between 20% and 50% equity ownership in a corporation or greater than 3% to 5% of a partnership interest. Strategic investments are held at cost, less impairment if any, when the Company is not considered to exert significant influence on operating and financial policies of the investee.

Prior to the Mergers, the Company had a strategic investment in Knight which was classified as available for sale securities. Any gain or losses in the fair value of this investment were recorded in Other comprehensive income (loss).

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 amortizes other intangible assets 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.

Discontinued operations and Assets and Liabilities

Revenues and expenses associated with a business line that has been disposed of through closure or sale, or are considered held for sale, are included in Loss from discontinued operations, net of tax on the Consolidated Statements of Operations. Assets and liabilities of a disposal group classified as discontinued operations are included in Assets within discontinued operations and Liabilities within discontinued operations, respectively, on the Consolidated Statements of Financial Condition. Cash flows from discontinued operations are presented on the Consolidated Statements of Cash Flows within operating, investing and financing activities, as applicable.

Payable to customers

Payable to customers arise primarily from futures transactions and include 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 or, in certain instances, as inducement grants to new hires.

Foreign currency translation and foreign currency forward contracts

The Company has foreign subsidiaries that do not utilize the U.S. dollar as their functional currency.

Assets and liabilities of the foreign subsidiaries having non-U.S. dollar functional currencies 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 (loss) income on the Consolidated Statements of Financial Condition and Cumulative translation adjustment on the Consolidated Statements of Comprehensive Income. Gains or losses resulting from foreign currency transactions are included in Investment income and other, net on the Company’s Consolidated Statements of Operations. For the three months ended September 30, 2013 and 2012, the Company recorded a loss of $0.1 million and a gain of $0.2 million, respectively. For the nine months ended September 30, 2013 and 2012, the Company recorded a loss of $1.3 million and a gain of $0.3 million, respectively.

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 (loss) income on the Consolidated Statements of Financial Condition and the Consolidated Statements of Comprehensive Income. 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 RSUs and are eligible to retire prior to the vesting of such awards.

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 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 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 estimated sub-lease income. In the event the Company is able to sublease the excess real estate after recording a lease loss, 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. In the event that the Company concludes that previously determined excess real estate is needed for the Company’s use, such lease loss accrual is adjusted accordingly.

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.

Variable interest entities

A VIE is an entity that lacks one or more of the following characteristics (i) the total equity investment at risk is sufficient to enable the entity to finance its activities independently and (ii) the equity holders have the power to direct the activities of the entity that most significantly impact its economic performance, the obligation to absorb the losses of the entity and the right to receive the residual returns of the entity. The Company has a controlling financial interest and will consolidate a VIE if it has both (i) the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance and (ii) the obligation to absorb losses of the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE.

VIEs generally finance the purchase of assets by issuing debt and equity securities that are either collateralized by or indexed to the assets held by the VIE. The debt and equity securities issued by a VIE may include tranches of varying levels of subordination. The Company’s involvement with VIEs includes purchased interests and commitments to VIEs.

The Company is principally involved with VIEs through Urban’s securitization of HECM loan inventory.  Urban sells HECM loans to GNMA trusts, which have the characteristics of a VIE, and Urban retains certain commitments and obligations to the GNMA trusts.  Urban's maximum exposure to loss is the value of its obligations as issuer and servicer of the GNMA trusts. Securitized HECM loan inventory variable interests are recorded in Assets and liabilities within discontinued operations.

The following table presents the Company’s nonconsolidated VIEs at September 30, 2013 (in thousands):

30-Sep-13

Securitized

HECM loan

inventory recorded within Assets within discontinued operations

Carrying value of the variable interests

Assets

$

—

Liabilities

1,326

Maximum exposure to loss in nonconsolidated VIEs

Commitments

43,932

Purchased interests

—

The Company did not have any nonconsolidated VIEs at December 31, 2012.

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 December 2011, the Financial Accounting Standards Board (“FASB”) issued an Accounting Standard Update (“ASU”) that requires additional disclosures about financial assets and liabilities that are subject to netting arrangements. Under the ASU, financial assets and liabilities must be disclosed at their respective gross asset and liability amounts, the amounts offset on the balance sheet and a description of the respective netting agreements. The new disclosures are required for reporting periods beginning on or after January 1, 2013, and are to be applied retrospectively. Other than requiring additional disclosures, the adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.

In February 2013, the FASB issued an ASU that requires additional disclosure requirements for items reclassified out of accumulated other comprehensive income. This new guidance requires entities to present either on the face of the income statement or in the notes to the financial statements the effects on the specific line items of the income statement for amounts reclassified out of accumulated other comprehensive income. This ASU is effective for reporting periods beginning after December 15, 2012. Other than additional disclosure requirements, the adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.

Recent accounting guidance to be adopted in future periods

In March 2013, the FASB issued an 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 is effective for reporting periods beginning after December 15, 2013, however early adoption is permitted. The Company is evaluating the impact of this ASU 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 net operating loss (“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 is required for reporting periods beginning after December 15, 2013. Upon adoption, entities are required to apply the provisions of the ASU prospectively for all unrecognized tax benefits that exist at the adoption date, however, the ASU also indicates that retrospective application is permitted. The Company is currently evaluating the impact that such adoption will have on its Consolidated Financial Statements.

Discontinued_Operations

Discontinued Operations	9 Months Ended
	Sep. 30, 2013
Discontinued Operations and Disposal Groups [Abstract]	'
Discontinued Operations	'
	Discontinued Operations
	In July 2013, the Company entered into an agreement to sell Urban, its reverse mortgage origination and securitization business that it acquired as a result of the Mergers, to an investor group. The transaction is expected to be completed in the fourth quarter of 2013. Completion of the sale is subject to certain customary conditions, including receipt of required regulatory approvals by GNMA, HUD, FNMA, and states and territories in which Urban operates and the absence of any law or order prohibiting the consummation of the sale of Urban. As a result, assets and liabilities related to Urban are classified as Assets within discontinued operations and Liabilities within discontinued operations on the Consolidated Statements of Financial Condition as of September 30, 2013 and the results of operations have been included in Loss from discontinued operations, net of tax within the Consolidated Statements of Operations for the three and nine months ended September 30, 2013.
	The revenues and results of operations of discontinued operations are summarized as follows (in thousands):
		For the three and nine months
		ended September 30,
		2013
	Revenues	$	22,114
	Expenses:
	Compensation	$	8,379
	Execution and clearance fees	2,840
	Payments for order flow	5,286
	Other expenses	6,873
	Total Expenses	23,378
	Pre-tax loss from discontinued operations	(1,264		)
	Income tax benefit	480
	Loss from discontinued operations, net of tax	$	(784	)
	Assets and liabilities within discontinued operations are presented in the following table (in thousands):
		September 30,
		2013
	Assets within discontinued operations:
	Cash and cash equivalents	$	15,702
	Securitized HECM loan inventory	5,841,664
	Loan inventory	177,674
	Receivable from brokers, dealers and clearing organizations	37,411
	Other assets	25,848
	Total assets	$	6,098,299
	Liabilities within discontinued operations:
	Liability to GNMA trusts	5,803,859
	Other secured financings	129,552
	Payable to brokers, dealers and clearing organizations	8,538
	Accrued compensation expense	4,717
	Accrued expenses and other liabilities	59,358
	Total liabilities	$	6,006,024
	The operations and cash flows of Urban will be eliminated from the ongoing operations of the Company as a result of the pending sale. The Company will not have significant continuing involvement in the operations of Urban after the sale is completed.

Assets_Segregated_or_Held_in_S

Assets Segregated or Held in Seperate Accounts Under Federal or Other Regulations	9 Months Ended
	Sep. 30, 2013
Cash and Cash Equivalents [Abstract]	'
Assets Segregated or Held in Seperate 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 primarily relate to the Company’s FCM business and consist of the following (in thousands):
		September 30,
		2013
	Cash and cash equivalents segregated under federal or other regulations	$	216,442
	Receivables from brokers, dealers and clearing organizations	282,650
	Total assets segregated or held in separate accounts under federal or other regulations	$	499,092

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2013
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
	30-Sep-13	Level 1				Level 2				Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,156,466			$	—			$	—		$	2,156,466
	Listed options	288,227				—				—			288,227
	U.S. government and Non-U.S. government obligations	32,678				—				—			32,678
	Corporate debt	46,606				—				—			46,606
	Total Financial instruments owned, at fair value	2,523,977				—				—			2,523,977
	Securities on deposit with clearing organizations (2)	160,369				—				—			160,369
	Deferred compensation investments (3)	—				84				—			84
	Investment in Deephaven Funds (3)	—				1,430				—			1,430
	Assets within discontinued operations:												—
	Securitized HECM loan inventory (4)	—				5,841,664				—			5,841,664
	Loan inventory	—				177,674				—			177,674
	Total fair value of financial instrument assets	$	2,684,346			$	6,020,852			$	—		$	8,705,198
	Liabilities
	Financial instruments sold, not yet purchased, at fair value:
	Equities (1)	$	1,848,729			$	—			$	—		$	1,848,729
	U.S. government obligations	29,441				—				—			29,441
	Corporate debt	49,616				—				—			49,616
	Listed options	229,721				—				—			229,721
	Foreign currency forward contracts	—				5,431				—			5,431
	Total Financial instruments sold, not yet purchased, at fair value	2,157,507				5,431				—			2,162,938
	Liabilities within discontinued operations:
	Liability to GNMA trusts, at fair value (4)	—				5,803,859				—			5,803,859
	Total fair value of financial instrument liabilities	$	2,157,507			$	5,809,290			$	—		$	7,966,797
	________________________________________
	(1)	Equities of $1.22 billion have been netted by their respective CUSIP number and their long and short positions.
	(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)	Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition.
	(4)	Represents HECMs that have been securitized into HECM Mortgage Backed Securities (“HMBS”) where the securitization is not accounted for as a sale of the underlying HECMs. See Securitized HECM loan inventory below for full description.
		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	31-Dec-12	Level 1				Level 2				Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	378,933			$	—			$	—		$	378,933
	Listed options	92,305				—				—			92,305
	U.S. government obligations and corporate bonds	68,765				—				—			68,765
	Mutual funds - Bond Funds	114,872								—			114,872
	Total Financial instruments owned, at fair value	654,875				—				—			654,875
	Securities on deposit with clearing organizations	7,147								—			7,147
	Investment - Knight preferred shares	199,632								—			199,632
	Total fair value of financial instrument assets	$	861,654			$	—			$	—		$	861,654
	Liabilities
	Financial instruments sold, not yet purchased, at fair value:
	Equities (1)	$	423,740			$	—			$	—		$	423,740
	U.S. government obligations	19,056				—				—			19,056
	Listed options	69,757				—				—			69,757
	Total Financial instruments sold, not yet purchased, at fair value	512,553				—				—			512,553
	Total fair value of financial instrument liabilities	$	512,553			$	—			$	—		$	512,553
	___________________________________
	(1) Equities of $5.9 million have been netted by their respective CUSIP number and their long and short positions.
	The Company’s equities, listed options, U.S. government and non-U.S. government obligations, and rated corporate debt 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.
	Certain instruments are classified within Level 3 of the fair value hierarchy because they trade infrequently and therefore have little or no price transparency. For those instruments that are not traded in active markets or are subject to transfer restrictions, valuations are adjusted to reflect illiquidity and/or non-transferability, and such adjustments are generally based on available market evidence. In the absence of such evidence, management’s best estimate is used.
	The Company’s assets measured at fair value on a nonrecurring basis solely relates to goodwill and intangible assets arising from various acquisitions which would be classified as Level 3 within the fair value hierarchy. See Footnote 10 “Goodwill and Intangible Assets” for additional information.
	There were no transfers of financial instruments between levels of the fair value hierarchy for any periods presented.
	The Company’s loan inventory, foreign currency forward contracts , deferred compensation investments and its 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 September 30, 2013, the Company had foreign currency forward contracts with a notional value of 80.0 million British pounds that are used to hedge the Company’s investment in its European subsidiaries. The fair value of these contracts was 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.
Loan inventory (within Assets within discontinued operations)
The Company’s loan inventory primarily comprises newly issued HECMs that Urban has originated or purchased and for which the Company has elected to account for at fair value. Significant inputs that are used in determining fair value include LIBOR and U.S. treasury interest rates, weighted average coupon and pricing of actively-traded HMBS and dealer quotations for HECMs. These instruments are recorded within Assets within discontinued operations on the Company's Consolidated Statements of Financial Condition.
Securitized HECM loan inventory (within Assets within discontinued operations)
Securitized HECM loan inventory comprises HECMs that Urban has securitized into HMBS. The Company has recorded the securitized loans within Assets within discontinued operations and a corresponding liability recorded as Liability to GNMA trusts, at fair value, within Liabilities within discontinued operations on its Consolidated Statements of Financial Condition. As of September 30, 2013 all of the HMBS created by the Urban have been sold to third parties. Significant inputs that are used in determining fair value include LIBOR and U.S. treasury interest rates, weighted average coupon and pricing of actively-traded HMBS and dealer quotations for HECMs.
Derivative Instruments
Fair value of derivative 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.
Futures
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 following tables summarize the fair value of derivative instruments and futures contract trading activities in the Consolidated Statements of Financial Condition and the gains and losses included in the Consolidated Statements of Operations (fair value in thousands):
			30-Sep-13
	Financial Statements		Assets							Liabilities
	Location		Fair Value				Contracts			Fair Value			Contracts
Foreign currency
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		$	21			651			$	62		244
Forward contracts (1)	Financial instruments owned, at fair value		—				—			5,431			2
Forward contracts (1)	Receivables from/Payables to brokers, dealers and clearing organizations		331				1			—			—
Equity
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		1,442				3,059			595			1,046
Swap contracts	Receivables from/Payables to brokers, dealers and clearing organizations		230				1			—			—
Listed options	Financial instruments owned, at fair value		288,227				772,768			229,721			753,979
Fixed income
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		4,183				24,400			1,724			6,601
Commodity
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		732				2,278			1,478			3,565
Total			$	295,166			803,158			$	239,011		765,437
(1) Designated as hedging instrument
			31-Dec-12
	Financial Statements		Assets							Liabilities
	Location		Fair Value				Contracts			Fair Value			Contracts
Foreign currency
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		$	8			500			$	—		95
Forward contracts	Accrued expenses and other liabilities		—				—			110			1
Equity
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		498				1,027			70			234
Swap contracts	Receivables from/Payables to brokers, dealers and clearing organizations		570				1			—			—
Listed options	Financial instruments owned, at fair value		92,305				199,324			69,757			196,804
Fixed income
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		3,474				42,143			5,043			61,398
Commodity
Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		428				1,385			410			1,824
Total			$	97,283			244,380			$	75,390		260,356
				Gain (Loss) Recognized
		Financial Statements		For the three months
				ended September 30,
		Location		2013				2012
Derivative instruments not designated as hedging instruments:
Foreign currency
Futures contracts		Trading revenues, net		$	3,825			$	4,493
Equity
Futures contracts		Trading revenues, net		17,461				10,724
Swap contracts		Trading revenues, net		1,884				1,985
Listed options (1)		Trading revenues, net		17,044				24,582
Fixed income
Futures contracts		Trading revenues, net		22,008				15,557
Commodity
Futures contracts		Trading revenues, net		18,774				8,792
				$	80,996			$	66,133
Derivative instruments designated as hedging instruments:
Foreign exchange - forward contract		Accumulated other comprehensive (loss)		$	(4,948	)		$	—
(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 revenue, net on the Company’s Consolidated Statements of Operations.
				Gain (Loss) Recognized
		Financial Statements		For the nine months ended September 30,
		Location		2013				2012
Derivative instruments not designated as hedging instruments:
Foreign currency
Futures contracts		Trading revenues, net		$	9,243			$	16,446
Equity
Futures contracts		Trading revenues, net		44,125				62,241
Swap contracts		Trading revenues, net		10,755				5,007
Listed options (1)		Trading revenues, net		63,322				41,455
Fixed income
Futures contracts		Trading revenues, net		64,211				66,573
Commodity
Futures contracts		Trading revenues, net		44,053				26,065
				$	235,709			$	217,787
Derivative instruments designated as hedging instruments:
Foreign exchange - forward contract		Accumulated other comprehensive (loss)		$	(4,948	)		$	—
(1)	Realized gains and losses on listed 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 revenue, net on the Company’s Consolidated Statements of Operations.
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):
	30-Sep-13	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts of Assets Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Financial Instruments		Cash Collateral Received
	Assets
	Listed options	$	288,227			$	—			$	288,227		$	—	$	—	$	288,227
	Securities borrowed	1,370,921				—				1,370,921			1,329,508		—		41,413
	Total Assets	$	1,659,148			$	—			$	1,659,148		$	1,329,508	$	—	$	329,640
	Liabilities
	Listed options	$	229,722			$	—			$	229,722		$	—	$	5,064	$	224,658
	Securities loaned	543,451				—				543,451			533,266		—		10,185
	Financial instruments sold under agreements to repurchase	595,029				—				595,029			595,022		—		7
	Foreign currency forward contracts	5,431				—				5,431			—		—		5,431
	Total Liabilities	$	1,373,633			$	—			$	1,373,633		$	1,128,288	$	5,064	$	240,281
	31-Dec-12	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts of Assets Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
													Financial Instruments		Cash Collateral Received
	Assets
	Listed options	$	92,305			$	—			$	92,305		$	—	$	—	$	92,305
	Securities borrowed	52,261				—				52,261			50,717				1,544
	Total Assets	$	144,566			$	—			$	144,566		$	50,717	$	—	$	93,849
	Liabilities
	Listed options	$	69,757			$	—			$	69,757		$	—	$	1,393	$	68,364
	Total Liabilities	$	69,757			$	—			$	69,757		$	—	$	1,393	$	68,364

Collateralized_Transactions

Collateralized Transactions	9 Months Ended
	Sep. 30, 2013
Collateralized Agreements [Abstract]	'
Collateralized Transactions	'
	Collateralized Transactions
	The Company receives financial instruments as collateral in connection with securities borrowed. Such financial instruments generally consist of equity and convertible securities but may include obligations of the U.S. government, federal agencies, foreign government and corporations. In most cases, the Company is permitted to deliver or repledge these financial instruments in connection with securities lending and other secured financings for meeting settlement requirements.
	The table below presents financial instruments at fair value received as collateral 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 counterparty (in thousands):
		September 30,		December 31,
		2013		2012
	Collateral permitted to be delivered or repledged	$	1,317,369	$	50,717
	Collateral that was delivered or repledged	1,210,408		50,717
	Collateral permitted to be further repledged by the receiving counterparty	149,891		—
	In order to finance securities positions and loan inventory, 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 and loans.
	The table below presents information about assets pledged by the Company (in thousands):
		September 30,		December 31,
		2013		2012
	Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge (1)	$	6,803,143	$	52,261
	Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	732,481		—
	(1)	Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge includes $5.84 billion recorded within Assets within discontinued operations as of September 30, 2013.

Receivable_from_and_Payable_to

Receivable from and Payable to Brokers, Dealers and Clearing Organizations	9 Months Ended
	Sep. 30, 2013
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):
		September 30,		31-Dec-12
		2013
	Receivable:
	Clearing organizations and other	$	1,096,308	$	140,089
	Securities failed to deliver	233,805		2,880
	Total Receivable	$	1,330,113	$	142,969
	Payable:
	Clearing organizations and other	$	564,592	$	15,879
	Securities failed to receive	101,586		8,306
	Total Payable	$	666,178	$	24,185
	Management believes that the carrying value of amounts receivable from and payable to brokers, dealers and clearing organizations approximates fair value since they are short term in nature.

Investments

Investments	9 Months Ended
	Sep. 30, 2013
Investments [Abstract]	'
Investments	'
	Investments
	Investments comprise strategic investments, including limited partnership investments, deferred compensation investments related to employee and director deferred compensation plans and investment in the Deephaven Funds. Investments consist of the following (in thousands):
		September 30,		December 31,
		2013		2012
	Strategic investments:
	Investments accounted for under the equity method	$	82,476	$	—
	Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	41,899		48,806
	Total Strategic investments	124,375		48,806
	Deferred compensation investments	84		—
	Knight preferred shares	—		199,632
	Investment in Deephaven Funds	1,430		—
	Total Investments	$	125,889	$	248,438

Goodwill_and_Intangible_Assets

Goodwill and Intangible Assets	9 Months Ended
	Sep. 30, 2013
Goodwill and Intangible Assets Disclosure [Abstract]	'
Goodwill and Intangible Assets	'
	Goodwill and Intangible Assets
	Goodwill is assessed for impairment annually or when events indicate that the amounts may not be recoverable. 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, $156.8 million and $13.8 million in identifiable intangible assets and goodwill, respectively, were recorded by the Company.
	No events occurred in the three or nine months ended September 30, 2013 or 2012 that would indicate that the carrying amounts of the Company’s goodwill or intangible assets may not be recoverable.
	The following table summarizes the Company’s Goodwill by segment (in thousands):
		September 30,				31-Dec-12
		2013
	Market Making	$	15,159			$	4,645
	Global Execution Services	3,239				—
	Total	$	18,398			$	4,645
	Intangible assets are amortized over their estimated remaining useful lives, the majority of which have been determined to range from four to 12 years. The weighted average remaining life of the Company’s intangible assets at September 30, 2013 and December 31, 2012 is approximately 7 years and 10 years, respectively.
	The following tables summarize the Company’s Intangible assets, net of accumulated amortization by segment and type (in thousands):
		September 30,				31-Dec-12
		2013
	Market Making
	Trading rights	$	47,065			$	42,635
	Technology	52,791				3,488
	Total	99,856				46,123
	Global Execution Services
	Customer relationships	34,139				—
	Trade names	3,900				—
	Technology	39,900				—
	Total	77,939				—
	Corporate and Other
	Technology	14,250
	Consolidated Total	$	192,045			$	46,123
			September 30,				31-Dec-12
			2013
	Customer relationships (1)	Gross carrying amount	$	35,000			$	—
		Accumulated amortization	(861		)		—
		Net carrying amount	34,139				—
	Trading rights (2)	Gross carrying amount	59,628				51,828
		Accumulated amortization	(12,563		)		(9,193		)
		Net carrying amount	47,065				42,635
	Trade names (3)	Gross carrying amount	4,000				—
		Accumulated amortization	(100		)		—
		Net carrying amount	3,900				—
	Technology (4)	Gross carrying amount	115,580				5,580
		Accumulated amortization	(8,639		)		(2,092		)
		Net carrying amount	106,941				3,488
	Total	Gross carrying amount	214,208				57,408
		Accumulated amortization	(22,163		)		(11,285		)
		Net carrying amount	$	192,045			$	46,123
	________________________________________
	(1)	Customer relationships relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of September 30, 2013. Lives may be reduced depending upon actual retention rates.
	(2)	Trading rights provide the Company with the rights to trade on certain exchanges. The weighted average remaining life is approximately 9 and 10 years as of September 30, 2013 and December 31, 2012, respectively.
	(3)	Trade names relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of September 30, 2013.
	(4)	The weighted average remaining life is approximately five and three years as of September 30, 2013 and December 31, 2012, respectively.
	The following table summarizes the Company’s amortization expense from continuing operations relating to Intangible assets (in thousands):
		For the three months								For the nine months
		ended September 30,								ended September 30,
		2013				2012				2013		2012
	Amortization expense	$	8,119			$	1,380			$	10,878	$	4,139
	As of September 30, 2013, the following table summarizes the Company’s estimated amortization expense for future periods (in thousands):
		Amortization
		expense
	For the three months ending December 31, 2013	$	8,119
	For the year ended December 31, 2014	32,477
	For the year ended December 31, 2015	31,780
	For the year ended December 31, 2016	31,082
	For the year ended December 31, 2017	31,082

LongTerm_Debt

Long-Term Debt	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Long-Term Debt	'
	Long-Term Debt
	The carrying value and fair value of the Company's Long-term debt is as follows (in thousands):
		30-Sep-13				31-Dec-12
		Carrying Amount		Fair Value		Carrying Amount		Fair Value
	Notes	$	—	$	—	$	15,000	$	15,000
	Senior Secured Notes	305,000		299,663		—		—
	Credit Agreement	535,000		535,000		—		—
	Convertible Notes	117,259		117,845		—		—
	Total	957,259		952,508		15,000		15,000
	Less: Current portion	235,000		235,000		—		—
	Total Long-term debt	$	722,259	$	717,508	$	15,000	$	15,000
	Notes Payable
	In October 2011, the Company issued $15.0 million in notes to a single lender. The notes bore interest at 5.95% per annum, required no principal amortization over the term and were scheduled to mature in October 2018. The note agreement included certain covenants which required the Company, among other things, to maintain compliance with debt to net worth ratios, maintain minimum levels of liquid net assets and maintain minimum net capital levels in regulated subsidiaries. At December 31, 2012, the Company was in compliance with these covenants. In connection with the Mergers, on May 31, 2013, the Company provided irrevocable notice to the lender to prepay the notes. The $15.0 million in notes were subsequently repaid on July 1, 2013 along with accrued interest and a $3.0 million early termination payment.
	Long-Term Debt in connection with Mergers
	In connection with the Mergers, KCG entered into a series of debt agreement transactions. Described below are the details of these transactions.
	Senior Secured Notes Indenture
	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, 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.
	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.
	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 September 30, 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, 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, (ii) issue exchange securities 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 accrue on the Senior Secured 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 provides for a future 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. For the three months ended September 30, 2013, there were no borrowings made against the $50.0 million first lien senior secured revolving credit facility.
	The First Lien Credit Facility bears 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 bear 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 bear 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. As of September 30, 2013, the interest rate was 5.75% per annum.
	The First Lien Credit Facility matures on December 5, 2017. The First Lien Credit Facility requires an amortization payment of $235.0 million on 7/1/2014 followed by quarterly amortization payments of $7.5 million on each September 30, December 31, March 31 and June 30, with the balance due on maturity.
	Optional prepayments of borrowings under the First Lien Credit Facility are permitted at any time, without premium or penalty, subject, however, to a 1% prepayment premium for optional prepayments of the First Lien Credit Facility made prior to July 1, 2014 with a new or replacement term loan facility with an “effective” interest rate less than that applicable to the First Lien Credit Facility.
	The First Lien Credit Facility is fully and unconditionally guaranteed on a joint and several basis by all of KCG's existing and future direct and indirect 100% owned domestic subsidiaries, other than certain subsidiaries including regulated broker-dealers and other regulated subsidiaries that, in each case, are not permitted to provide such guarantees under applicable law.
	The First Lien Credit Facility 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, capital expenditures and issuance of capital stock. It also contains financial maintenance covenants establishing a maximum consolidated first lien leverage ratio, a minimum consolidated interest coverage ratio and a minimum consolidated tangible net worth.
	The Company incurred issuance costs of $34.6 million in connection with the issuance of Senior Secured Notes and Credit Agreement. The issuance costs are recorded within Other assets on the Consolidated Statements of Financial Condition and are amortized over the respective terms of the Senior Secured Notes and the Credit Agreement. Including issuance costs, the Senior Secured Notes and Credit Agreements have effective yields of 9.0% and 7.6%, respectively. Subsequent to September 30, 2013, the Company made a $200.0 million principal prepayment under the Credit Agreement. See Footnote 20 "Subsequent Events" for further discussion.
	Revolving Credit Agreement
	On July 1, 2013, OCTEG, LLC (“OCTEG”), and Knight Capital Americas LLC ("KCA"), each of which are wholly-owned broker-dealer subsidiaries of KCG effective July 1, 2013, as borrowers, and KCG, as guarantor, entered into a credit agreement (the “OCTEG-KCA Facility Agreement”) with a consortium of banks and financial institutions. The OCTEG-KCA Facility Agreement replaces an existing credit agreement, dated as of June 6, 2012, among OCTEG and three banks.
	The OCTEG-KCA 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 “OCTEG-KCA Revolving Facility”): Borrowing Base A and Borrowing Base B. The OCTEG-KCA 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.
	Borrowings under the OCTEG-KCA Facility shall bear interest, at the applicable 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 September 30, 2013, there were no outstanding borrowings under the OCTEG-KCA Facility Agreement.
	The borrowers will be charged a commitment fee at a rate of 0.35% per annum on the average daily amount of the unused portion of the OCTEG-KCA Facility Agreement.
	The loans under the OCTEG-KCA Facility Agreement will mature on 6/6/2015. The OCTEG-KCA Revolving Facility is fully and unconditionally guaranteed on an unsecured basis by KCG and, to the extent elected by OCTEG or KCA, 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 OCTEG-KCA Revolving Facility 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 each of OCTEG and KCA, a maximum total asset to total regulatory capital ratio for each of OCTEG and KCA, a minimum excess net capital limit for each of OCTEG and KCA, a minimum liquidity ratio for KCA, and a minimum tangible net worth threshold for KCG. As of September 30, 2013, the Company was in compliance with the covenants.
	In connection with the OCTEG-KCA 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 is being amortized over the term of the OCTEG-KCA Revolving Facility.
	OCTEG and KCA shall each be obligated only with respect to the principal and interest of their own borrowings, and not the interest and principal of the other borrower's borrowings.
	Cash Convertible Senior Subordinated Notes
	In March 2010, Knight issued $375.0 million 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 Long-term debt in the Company’s Consolidated Statements of Financial Condition.
	As a result of the Mergers, on July 1, 2013, KCG became a party to the Company's existing $375.0 million Convertible Notes. 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 KCG Class A Common Stock to receive cash consideration for such KCG Class A Common Stock constitutes a “Fundamental Change";
	•	Each holder of the Convertible Notes has the right to deliver a “Fundamental Change Repurchase Notice ” requiring Knight 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 Knight's Convertible Notes in full at maturity, was deposited in a cash collateral account under the sole dominion and control of the collateral agent under the First Lien Credit Facility (the "Collateral Account").
	On 8/5/2013, 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. At 9/30/2013, $117.3 million remained in the Collateral Account which is included in Other assets on the Consolidated Statements of Financial Condition. See Footnote 20 "Subsequent Events" for a discussion of the consent solicitation related to the Collateral Account.
	The Company recorded expenses with respect to the Long term debt as follows (in thousands):
		For the three months				For the nine months
		ended September 30,				ended September 30,
		2013		2012		2013		2012
	Interest expense	$	19,173	$	688	$	21,366	$	1,134
	Amortization of debt issuance cost (1)	2,007		103		2,479		152
	Commitment fee (1)	267		390		9,204		390
	Total	$	21,447	$	1,181	$	33,049	$	1,676
	(1)	Included in Other expense.

StockBased_Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2013
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"). At such time, the Knight Capital Group, Inc. 2009 Executive Incentive Plan was also assumed by KCG in connection with the Mergers and was renamed the KCG Holdings, Inc. Amended and Restated Executive Incentive Plan. As of July 1, 2013, the number of shares reserved for issuance under the KCG Plan was 20.7 million (based on a conversion ratio of one third of a share of KCG Class A Common Stock for each share of Knight Class A Common Stock).
	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 through June 30, 2013.
	As a result of the Mergers, each Knight restricted share granted after December 19, 2012 and each outstanding Knight RSU was replaced with a restricted share or restricted stock unit, as applicable, equal to one third of a share of common stock of KCG (rounded to the nearest whole share). Knight awards granted on or prior to December 19, 2012 (except for RSUs that vest based upon performance) automatically vested upon the completion of the Mergers. Knight awards granted after December 19, 2012 (and RSUs granted on or prior to December 19, 2012 that vest based on performance) will 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. 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, primarily recorded in Employee compensation and benefits, and the corresponding income tax benefit, which was recorded in Income tax benefit on the Consolidated Statements of Operations are presented in the following table (in thousands):
		For the three months
		ended September 30,
		2013
	Stock award compensation expense	$	12,234
	Income tax benefit	4,649
	The following table summarizes restricted awards activity, including awards related to employees working in businesses that are included within discontinued operations, for the three months ended September 30, 2013 (awards in thousands):
			Restricted Stock Units
			Number of			Weighted-
			Units			Average
						Grant date
						Fair Value
	1-Jul-13		—			$	—
	Conversion of outstanding Knight RSUs		3,251			11.22
	Granted		6,004			10.58
	Vested		(45	)		11.07
	Forfeited		(246	)		11.45
	Outstanding at September 30, 2013		8,963			$	10.78
	There is $79.6 million of unamortized compensation related to the unvested restricted awards outstanding at September 30, 2013. The cost of these unvested restricted shares is expected to be recognized over a weighted average life of 2.5 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 at not less than market value. 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 continue to vest following certain termination circumstances, in accordance with the applicable award agreements. 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 stock plans. 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.
	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 on the Consolidated Statements of Operations are as follows (in thousands):
		For the three months
		ended September 30,
		2013
	Stock option and SAR compensation expense	$	915
	Income tax benefit	348
	The following table summarizes stock option and SAR activity and stock options exercisable for the three months ended September 30, 2013 (awards in thousands):
		Number of Stock Awards			Weighted-				Aggregate				Weighted-
					Average				Intrinsic				Average
					Exercise				Value				Remaining
					Price								Life (years)
	1-Jul-13	—			$	—
	Conversion of outstanding Knight options	782			41.25
	Granted at market value (1)	4,324			15.19
	Exercised	—			—
	Forfeited or expired	(79	)		45.81
	Outstanding at September 30, 2013 (1)	5,027			$	18.76			$	387			4.49
	Exercisable at September 30, 2013	703			$	40.74			$	381			2.61
	Available for future grants at September 30, 2013 *	6,327
	* Represents both options and awards available for grant.
	(1) Includes 1.7 million of SARs.
	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 exercised during the three months ended September 30, 2013.
	There is $7.2 million unamortized compensation related to unvested stock options and SARs outstanding at September 30, 2013. The cost of these unvested awards is expected to be recognized over a weighted average life of 2.2 years.
	Incentive units
	Prior to the Mergers, GETCO awarded deferred compensation to its employees in the form of incentive units that generally vest over time. The value of these incentive units is determined based on the same methodology used to value the GETCO Class B unit awards and the amount expensed is 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. Given that the units vested in connection with the Mergers, the Company fully amortized the units as of June 30, 2013. The accelerated amortization of incentive units recorded during the nine months ended September 30, 2013 was $1.3 million. Deferred compensation payable at September 30, 2013 and December 31, 2012 related to incentive units was $3.0 million and $3.5 million, respectively, and is included in accounts Accrued compensation expense on the Consolidated Statements of Financial Condition.
	The following is a summary of the changes in the incentive units for the nine months ended September 30, 2013 (units in thousands):
		Vested			Unvested
	Incentive units at January 1, 2013	24			45
	Issued	1			12
	Vested	53			(53	)
	Exercised	(22	)		—
	Canceled	(1	)		(4	)
	Incentive units at September 30, 2013	55			—
Class B units
Prior to the Mergers, the Company granted membership unit awards to employees in the form of Class B units. Prior to 2012, these primarily consisted of non-voting units which vest three years from the grant date, provided certain conditions of employment and performance were met by the employee. In 2012, the Company changed the vesting of units granted in 2012 to an annual vesting of one-third of the units over the 3 year period. Upon termination of employment, the Company 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. The Company 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. The accelerated amortization of Class B units recorded during the nine months ended September 30, 2013 was $9.4 million.
The following is a schedule of the changes in the Company’s unvested Class B units (units in thousands):
		Units			Weighted Average Grant Price
Unvested as of December 31, 2012		417			$	89.99
Issued		58			73.54
Vested		(438	)		87.86
Forfeited		(37	)		89.28
Unvested as of September 30, 2013		—			$	—
Class E units
In 2012, the Company also granted employees profit interests in the form of Class E units. Prior to 2012, Class E units primarily vested three years from the grant date. For units granted in 2012, the Company changed the vesting of Class E units to an annual vesting of one-third of the units over the three year period and provided the Company an option to repurchase the units at the end of 5 years. Class E units allowed for future appreciation in excess of the Company’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. The Company 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. The accelerated amortization of the Class E units recorded during the three and nine months ended September 30, 2013 was $0 and $3.5 million, respectively.
The following is a schedule of the changes in the Company’s unvested Class E units (units in thousands):
		Units			Weighted Average Grant Price
Unvested as of December 31, 2012		476			$	36.78
Issued		—			—
Vested		(442	)		37.24
Forfeited		(34	)		30.89
Unvested as of September 30, 2013		—			$	—
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):
		Three months ended September 30,								Nine months ended September 30,
		2013				2012				2013			2012
Class B and E units		$	—			$	5,429			$	19,860		$	9,124
Incentive units		(875		)		368				1,428			788
Total		$	(875	)		$	5,797			$	21,288		$	9,912

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	Following the Mergers, the Company and its subsidiaries will file a consolidated federal income tax return as well as combined state income tax returns in certain jurisdictions. In other jurisdictions, the Company and its subsidiaries will file separate company state and local income tax returns. 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.
	Upon completion of the Mergers, the Company became subject to U.S. corporate income taxes. As described in Footnote 2 “Merger of GETCO and Knight”, following the Mergers the Company recorded $65.9 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 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 following table reconciles the U.S. federal statutory income tax to the Company's actual income tax from continuing operations (in thousands):
		For the three months						For the nine months
		ended September 30,						ended September 30,
		2013			2012			2013			2012
	U.S. federal statutory income tax (benefit) expense	$	41,949		$	5,114		$	12,543		$	12,224
	Income not subject to U.S. corporate income tax	(44,790		)	(3,515		)	(14,071		)	(9,801		)
	U.S. state and local income taxes, net of U.S. federal income tax effect	(391		)	533			82			825
	Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	(103,499		)	—			(103,499		)	—
	Nondeductible charges (1)	(1,114		)	74			(944		)	166
	Foreign taxes	—			2,581			3,559			6,958
	Other, net	78			18			(148		)	(4		)
	Income tax (benefit) expense	$	(107,767	)	$	4,805		$	(102,478	)	$	10,368
	(1) Nondeductible charges include nondeductible compensation and meals and entertainment.
	Deferred income taxes reflect the net tax effects of temporary differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when such differences are expected to reverse. Valuation allowances recorded on the balance sheet dates are necessary in cases where management believes that it is more likely than not that some portion or all of the deferred tax assets will not be realized.
	Based on the weight of the positive and negative evidence considered, management believes that it is more likely than not that the Company will be able to realize its federal deferred tax assets in the future, and therefore no valuation allowance has been recorded at September 30, 2013. Management believes that positive evidence including the Company's history of sustainable profitability, and its forecasts of future profitability outweighs the negative evidence. Also, a significant portion of Knight’s customer base has remained unchanged and the overall businesses acquired as a result of the Mergers are profitable, and the Company does not expect losses to recur. The Company has recorded a full 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.
	Included in the Company’s deferred tax assets are benefits associated with NOL carryforwards generated by Knight in periods prior to the Mergers. At September 30, 2013, the Company had projected overall U.S. federal NOL carryforwards of $89.2 million. The Company recorded a related deferred income tax asset for its NOLs of $31.2 million as of September 30, 2013, and an offsetting valuation allowance of $6.8 million 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 September 30, 2013 the Company recorded a valuation allowance for substantially all of its state and local NOL carryforwards 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. Certain of these carryforwards are subject to annual limitations on utilization and they will begin to expire in 2019.
	At September 30, 2013, the Company had non-U.S. NOL carryforwards of $68.7 million which were generated by Knight in periods prior to the Mergers. The Company recorded a foreign deferred income tax asset of $17.9 million for these NOL carryforwards as of September 30, 2013, along with an offsetting U.S. federal deferred tax liability of $17.9 million for the expected future reduction in U.S. foreign tax credits associated with the use of the non U.S. loss carryforwards. These non-U.S. net operating losses may be carried forward indefinitely. At September 30, 2013 the Company had tax credit carryforwards which were generated by Knight in periods prior to the Mergers, comprising foreign tax credit carryforwards of $3.2 million (and an offsetting valuation allowance of $1.6 million), general business credit carryforwards of $2.5 million and alternative minimum tax credit carryforwards $6.8 million.
	At September 30, 2013, the Company had $1.4 million of unrecognized tax benefits, all of which would affect the Company's effective tax rate if recognized.
	As of September 30, 2013, the Company is subject to U.S. Federal income tax examinations for the tax years 2009 through 2012, and to non U.S. income tax examinations for the tax years 2007 through 2012. In addition, the Company is subject to state and local income tax examinations in various jurisdictions for the tax years 2007 through 2012. The final outcome of these examinations is not yet determinable. However, the Company anticipates that adjustments to the unrecognized tax benefits, if any, will not result in a material change to the results of operations or financial condition.
	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 Interest expense and Interest, net, on the Consolidated Statements of Operations.

Accumulated_Other_Comprehensiv

Accumulated Other Comprehensive Loss	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Accumulated Other Comprehensive Loss	'
	Accumulated Other Comprehensive Loss
	The following table presents changes in Accumulated other comprehensive loss, net of tax by component for the nine months ended September 30, 2013 (in thousands):
		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments			Total
	Balance January 1, 2013	$	114,319		$	—		$	114,319
	Other comprehensive income	13,604			(5,098		)	8,506
	Reclassification of gain on investment in Knight Common Stock	(127,972		)	—			(127,972		)
	Net current-period other comprehensive loss	(114,368		)	(5,098		)	(119,466		)
	Balance September 30, 2013	$	(49	)	$	(5,098	)	$	(5,147	)

Earnings_Per_Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2013
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 options, warrants and SARs excluded was approximately 28.8 million for the three and nine months ended September 30, 2013. 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 three and nine months ended September 30, 2013 and 2012 (in thousands):
		For the three months ended September 30,
		2013				2012
		Numerator /		Denominator /		Numerator /		Denominator /
						net
		net income		shares		income		shares
	Income from continuing operations and shares used in basic calculations	$	227,621	114,113		$	9,807	46,411
	Effect of dilutive stock based awards			660				—
	Income from continuing operations and shares used in diluted calculations	$	227,621	114,773		$	9,807	46,411
	Basic earnings per share from continuing operations			$	1.99			$	0.21
	Diluted earnings per share from continuing operations			$	1.98			$	0.21
		For the nine months ended September 30,
		2013				2012
		Numerator /		Denominator /		Numerator /		Denominator /
						net
		net income		shares		income		shares
	Income from continuing operations and shares used in basic calculations	$	138,316	68,632		$	24,556	49,619
	Effect of dilutive stock based awards			223				—
	Income from continuing operations and shares used in diluted calculations	$	138,316	68,855		$	24,556	49,619
	Basic earnings per share from continuing operations			$	2.02			$	0.49
	Diluted earnings per share from continuing operations			$	2.01			$	0.49
	The number of shares used to calculate EPS for 2012 are GETCO units converted into KCG shares using an exchange ratio as detailed in the Merger Agreement.

Significant_Clients

Significant Clients	9 Months Ended
	Sep. 30, 2013
Risks and Uncertainties [Abstract]	'
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 dollar value traded by the Company. No clients accounted for more than 10% of the Company’s U.S. equity dollar value traded for the three and nine months ended September 30, 2013 or 2012.

Commitments_and_Contingent_Lia

Commitments and Contingent Liabilities	9 Months Ended
	Sep. 30, 2013
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 Knight 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. 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 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 period, depending, in part, upon operating results for that period.
	As previously disclosed in 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 the Company's broker-dealer subsidiary, KCA, sending numerous erroneous orders in NYSE-listed and NYSE Arca securities into the market. As noted in Knight's Form 10-K for the year ended December 31, 2012, Knight has since been named in two putative class actions and one derivative lawsuit relating to the technology issue and has 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 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 allege, 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, seek 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 possible litigation related to the technology issue, the August 6, 2012 recapitalization, or the Mergers, an unfavorable outcome in one or more of these matters could have a material adverse effect on the Company's financial condition or ongoing results of operations. In addition, the Company expects to incur additional expenses in defending against such litigation.
	Legal
	On October 26, 2012, Knight, its Chairman and Chief Executive Officer, Thomas M. Joyce, and its 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. Generally, this putative class action complaint alleges that the defendants made material misstatements and/or failed to disclose matters related to the events of August 1. The plaintiff asserts claims under Sections 10(b) and 20 and Rule 10b-5 of the federal securities laws, claiming that he and a 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. More specifically, the plaintiff alleges that 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 asserts claims under Sections 10(b) and 20 and Rule 10b-5 of the federal securities laws, claiming that it and a class of Knights 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. The motion to dismiss was fully briefed as of August 2013, and the Company is awaiting a decision.
	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 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.
	As previously disclosed in the Current Report on Form 8-K filed by Knight on June 14, 2013, 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 contemplates that the parties will enter into a stipulation of settlement. The stipulation of settlement will be subject to customary conditions, including court approval following notice to Knight's former stockholders. In the event that the parties enter into a stipulation of settlement, a hearing will be scheduled at which the Delaware Court of Chancery will consider the fairness, reasonableness and adequacy of the settlement. If the settlement is finally approved by the court, it will 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 contemplate 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. There can be no assurance that the parties will enter into a stipulation of settlement, or that the court will approve any proposed settlement. In such event, the proposed settlement as contemplated by the memorandum of understanding may be terminated.
	The settlement is not, and should not be construed as, an admission of wrongdoing or liability by any of the defendants. The defendants continue to believe that the shareholder actions challenging the Mergers are without merit and vigorously deny the allegations that Knight's directors breached their fiduciary duties. Likewise, defendants do not believe that any disclosures regarding the Mergers are required under applicable laws other than that which have already been provided in the Proxy Statement. Nonetheless, the defendants entered into the memorandum of understanding to avoid the risk of the putative stockholder class action 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 shareholder actions.
	Regulatory
	As previously discussed in Knight's public filings, on August 9, 2012, the SEC began an examination of KCA's compliance with SEC Rule 15c3-5 (the “Market Access Rule”) and other rules and regulations as they relate to the August 1, 2012 technology issue. The SEC issued a formal order of investigation concerning Knight and KCA on August 29, 2012.
	On October 16, 2013, KCA reached a settlement with the SEC relating to the August 1, 2012 technology issue as described further in Footnote 20 "Subsequent Events".
	The full amount of this settlement had been accrued on Knight's July 1, 2013 balance sheet acquired by the Company.
	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 and reverse mortgage originator, 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. In some instances, these matters may rise to a disciplinary action and/or civil or administrative action.
	In the second quarter of 2012, Knight recorded pre-tax trading losses of $35.4 million related to the Facebook IPO. On August 1, 2012 Nasdaq’s proposed voluntary accommodation program (the “Accommodation Program”) was published in the Federal Register by the SEC. The Accommodation Program creates a fund for voluntary accommodations for qualifying Nasdaq members disadvantaged by problems that arose during the Facebook IPO. Under the Accommodation Program as proposed by Nasdaq, Knight would recover a portion of its pre-tax trading losses. The Accommodation Program was approved by the SEC on March 22, 2013 which would allow Nasdaq to compensate market participants for certain claims related to system difficulties in connection with the Facebook IPO in an amount not to exceed $62.0 million. On April 2, 2013, Knight submitted an accommodation claim to Nasdaq totaling approximately $13.0 million comprising approximately $2.6 million which would be reimbursed to customers and approximately $10.4 million to be retained by Knight (related to customer trades which had previously been reimbursed to customers by Knight). On October 25, 2013, Nasdaq provided the Company with notice that it had completed its review of the Company’s accommodation claim.  In its notice, Nasdaq provided the details relating to its analysis and the amounts that Nasdaq was prepared to compensate the Company. The amount proposed by Nasdaq was approximately 17% less than the claimed amount submitted by the Company under the Accommodation Program.  The Company is in the process of reviewing the details of Nasdaq’s analysis.
	Lease and Contract Obligations
	Capital Leases
	During 2012, the Company entered into capitalized lease obligations related to certain computer equipment. These obligations represent drawdowns under a revolving secured lending facility with a single lender. At September 30, 2013, the obligations have a weighted-average interest rate of 3.85% 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 September 30, 2013 consist of (in thousands):
			Minimum Payments
	Through December 31, 2013		$	2,885
	2014		8,222
	2015		2,072
	Total		$	13,179
	The total interest expense related to capital leases for the three and nine months ended September 30, 2013 and 2012 included in the Consolidated Statements Operations is as follows (in thousands):
		Three months ended September 30,					Nine months ended September 30,
		2013			2012		2013		2012
	Interest expense - Capital leases	$	151		$	285	$	602	$	1,135
	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 $5.6 million and $2.3 million for the three months ended September 30, 2013 and 2012, respectively, and is included in Occupancy and equipment rentals on the Consolidated Statements of Operations. Rental expense from continuing operations under the office leases was $10.4 million, and $6.4 million for the nine months ended September 30, 2013 and 2012, 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 September 30, 2013, 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
	Three months ending December 31, 2013	$	8,014		$	525	$	7,489
	Year ending December 31, 2014	30,170			1,868		28,302
	Year ending December 31, 2015	28,346			1,494		26,852
	Year ending December 31, 2016	27,877			1,464		26,413
	Year ending December 31, 2017	27,262			1,541		25,721
	Thereafter through December 31, 2027	108,318			358		107,960
	Total	$	229,987		$	7,250	$	222,737
	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 September 30, 2013, the Company had provided a letter of credit for $1.0 million, collateralized by U.S. Treasury Bills, as a guarantee for one of the Company’s lease obligations. 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.
	Urban has floating rate HECMs which provide borrowers additional borrowing capacity of approximately $726.0 million as of September 30, 2013. This additional borrowing capacity is primarily in the form of undrawn lines of credit, with the balance generally available on a scheduled payments basis.

Financial_instruments_with_Off

Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk	9 Months Ended
	Sep. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Financial instruments with off-balance sheet risk and concentrations of credit risk	'
	Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk
	As a market maker of equities and options, 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.
	The Company, through its FCM, also provides execution and clearing services in futures and options on futures contracts to facilitate customer transactions on major U.S. and European futures and options exchanges. Customer activities may expose the Company to off-balance sheet risk in the event the FCM customer is unable to fulfill its contracted obligation as the Company guarantees the performance of its clients to the respective clearing houses or other brokers. In accordance with regulatory requirements and market practice, the Company requires its customers to meet, at a minimum, the margin requirements established by each of the exchanges at which contracts are traded. Margin is a good faith deposit from the customer that reduces risk to the Company of failure by the customer to fulfill obligations under these contracts. The Company establishes customer credit limits and monitors required margin levels on a daily basis and, pursuant to such guidelines, require customers to deposit additional collateral, or to reduce positions, when necessary. Further, the Company seeks to reduce credit risk by entering into netting agreements with customers, which permit receivables and payables with such customers to be offset in the event of a customer default. Management believes that the margin deposits and collateral held at September 30, 2013 were adequate to minimize the risk of material loss that could be created by positions held at that time.
	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	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Business Segments	'
	Business Segments
	As of September 30, 2013, the Company's operating segments comprised the following: (i) Market Making; (ii) Global Execution Services; and (iii) Corporate and Other.
	The Market Making segment principally consists of market making in the cash, futures and/or options markets across global equities, fixed income, commodities and foreign currencies. As a market maker, the Company commits capital for trade executions by offering to buy securities from, or sell securities to, institutions and broker-dealers. The Market Making segment primarily includes client direct and exchange-based electronic market making activities, including trade executions as an equities DMM on the NYSE and NYSE Amex. The Company is an active participant in all major global equity and futures exchanges and also trades on substantially all domestic electronic options exchanges. As a complement to electronic market making, the Company’s cash trading business handles specialized orders and also transacts on the OTC Bulletin Board, marketplaces operated by the OTC Markets Group Inc. and the AIM of the London Stock Exchange.
	The Global Execution Services segment provides agency execution services and operates trading venues. This segment offers access via its electronic agency-based platforms to markets and self-directed trading in equities, options, fixed income, foreign exchange and futures. In contrast to Market Making, the Global Execution Services segment generally does not act as a principal to transactions that are executed within this segment; however, it will commit capital on behalf of clients as needed, and generally earns commissions for acting as agent between the principals to the trade. Global Execution Services includes high touch equity sales and trading (including exchange traded funds ("ETFs")) aided by its network of sales employees. This segment also facilitates client orders through program, block, and riskless principal trades. Additionally, the Global Execution Services segment includes the FCM business, which provides clients with futures execution and clearing services on major U.S. and European futures and options exchanges.
	The Corporate and Other segment invests in strategic financial services-oriented opportunities, allocates, deploys and monitors all capital, and maintains corporate overhead expenses and all other income and expenses that are not attributable to the other segments, including the gain recognized on Knight Common Stock. The Corporate and Other segment also houses 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 three months ended September 30, 2013:
	Revenues	$	240,110	$	91,366		$	136,343	$	467,819
	Pre-tax earnings	47,853		(16,354		)	88,355		119,854
	Total assets	3,882,763		1,426,824			1,885,408		7,194,995
	For the three months ended September 30, 2012:
	Revenues	$	107,672	$	9,258		$	13,672	$	130,602
	Pre-tax earnings	8,214		(1,120		)	7,518		14,612
	Total assets	1,507,881		25,934			239,490		1,773,305
	For the nine months ended September 30, 2013:
	Revenues	$	455,678	$	113,701		$	128,744	$	698,123
	Pre-tax earnings	55,608		(21,248		)	1,478		35,838
	Total assets	3,882,763		1,426,824			1,885,408		7,194,995
	For the nine months ended September 30, 2012:
	Revenues	$	383,203	$	27,248		$	14,790	$	425,241
	Pre-tax earnings	37,976		(3,839		)	787		34,924
	Total assets	1,507,881		25,934			239,490		1,773,305
	Total Assets for September 30, 2013 do not include Assets within discontinued operations of $6.1 billion.

Subsequent_Events

Subsequent Events	9 Months Ended
	Sep. 30, 2013
Subsequent Events [Abstract]	'
Subsequent Events	'
	Subsequent Events
	Consent Solicitation and Credit Agreement Amendment
	On September 30, 2013, the Company commenced a consent solicitation with respect to its outstanding Senior Secured Notes. The Company solicited consents from registered holders (“Holders”) of the Senior Secured Notes 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. The amendment was sought to permit the Company to use the funds on deposit in the Collateral Account previously set aside to purchase the Convertible Notes to repay a portion of the principal amount of the borrowings under the Credit Agreement.
	On October 15, 2013, the Company announced that it had received consents from the Holders of $304.0 million in aggregate principal amount of the Senior Secured Notes, representing 99.68% of the total aggregate principal amount, which met the consent threshold requirement under the Senior Secured Notes Indenture.
	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 prior to maturity 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 connection with the consent solicitation and the Third Supplemental Indenture, the Company entered into an Amendment to its Credit Agreement with the consent of the requisite percentage of lenders under its First Lien Credit Facility. The Credit Agreement Amendment permits the Company to prepay a portion of the principal amount of borrowings under the First Lien Credit Facility from time to time for a period of 60 days following the effective date of the Credit Agreement Amendment out of the cash set aside in a cash collateral account under the sole dominion and control of the collateral agent under the First Lien Credit Facility for repayments of the Convertible Notes, after which period the remaining cash in that cash collateral account will be required to be used to prepay a portion of the principal amount of borrowings under the First Lien Credit Facility. The Credit Agreement Amendment also permits the purchase, redemption or repayment prior to maturity of the Convertible Notes at any price, including at a premium or at a discount from the face value thereof, with any available cash. See “Principal prepayment under Credit Agreement” for a discussion of the prepayment.
	In conjunction with the consent solicitation, the Company paid $2.8 million to the Holders and its investment bank. This amount will be included within Other assets on the Consolidated Statements of Financial Condition and will be amortized over the remaining life of the Senior Secured Notes.
	Principal prepayment under Credit Agreement
	On October 23, 2013, the Company made a $200.0 million principal prepayment under the Credit Agreement. Under the terms of the Credit Agreement, an amortization payment of $235.0 million is due on July 1, 2014, followed by amortization payments of $7.5 million each quarter beginning September 30, 2014 until maturity on December 5, 2017. The $200.0 million principal prepayment reduces the first $235.0 million amortization payment under the First Lien Credit Facility to $35.0 million.
	A portion of the $200.0 million was drawn from $117.8 million in cash held in the Collateral Account, which was recorded in Other assets on the Consolidated Statements of Financial Condition at September 30, 2013. As mentioned above, the Company secured the consents necessary from creditors to use the cash in the cash collateral account for the repayment. The remainder of the $200.0 million was paid out of available cash.
	Settlement with SEC
	As mentioned in Footnote 17 "Commitments and Contingent Liabilities", on October 16, 2013, KCA reached a settlement with the SEC relating to the August 1, 2012 technology issue.
	KCA, without admitting or denying the findings, consented to the issuance of an administrative Order relating to controls and procedures required by Section 15(c)(3) of the Securities Exchange Act of 1934 and SEC Rule 15c3-5 (the “Market Access Rule”), and Rules 200(g) and 203(b) of Regulation SHO (the “Short Sale Rules”). Under the terms of the settlement, KCA was censured, paid a civil penalty in the sum of $12.0 million and is required to cease and desist from committing future violations of the Market Access Rule and the Short Sale Rules. KCA must also retain an independent consultant to conduct a comprehensive review of KCA’s compliance with the Market Access Rule. The settlement resolves the SEC’s investigation relating to the August 1, 2012 technology issue. The full amount of this settlement had been accrued on Knight's July 1, 2013 balance sheet acquired by the Company and included in Accrued expenses and other liabilities on the September 30, 2013 Consolidated Statements of Financial Condition. The foregoing description of the settlement is qualified in its entirety by the full text of the SEC’s Order which is incorporated by reference herein to Exhibit 10.1 of the Current Report on Form 8-K filed by the Company on October 16, 2013.
	Reduction in force
	On October 22, 2013, in connection with post-closing merger integration and rationalization of redundancies across the combined firms, the Company commenced a workforce reduction plan. The workforce reduction plan impacted approximately 5% of the Company’s global workforce, all of whom have been notified. The Company currently estimates that it will recognize a pre-tax charge in connection with the workforce reduction of approximately $9.4 million during the fourth quarter of 2013. The charge is expected to consist of severance, other one-time termination benefits, and other associated costs.

Significant_Accounting_Policie1

Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2013

Accounting Policies [Abstract]

'

Basis of consolidation and form of presentation

'

Basis of consolidation and form of presentation

The accompanying unaudited Consolidated Financial Statements, prepared in conformity with GAAP, include the accounts of the Company and its subsidiaries and should be read in conjunction with the audited consolidated financial statements of GETCO for the year ended December 31, 2012 contained in the Current Report on Form 8-K filed by the Company on November 12, 2013 and in the Current Report on Form 8-K filed by the Company on July 1, 2013 and with the audited consolidated financial statements of Knight for the year ended December 31, 2012 in the Current Report on Form 8-K filed by Knight on May 13, 2013. 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.

The Company consolidates all of its subsidiaries as well as any variable interest entity (“VIE”) for which it is considered to be the primary beneficiary. The Company performs a qualitative assessment to determine if a VIE should be consolidated. As described in more detail below, the primary attributes the Company assesses include the entity’s capital structure and power. The Company will consolidate a VIE if it has both i) the power to direct the activities of a VIE that most significantly impact the entity’s economic performance and ii) the obligation to absorb losses of the entity that could potentially be significant to the VIE or the right to receive benefits from the entity that could potentially be significant to the VIE. As of September 30, 2013 and December 31, 2012, the Company was not considered to be a primary beneficiary of any VIE.

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.

Cash and securities segregated under federal and other regulations

'

Cash and securities segregated under federal and other regulations

The Company maintains custody of customer funds and, as a result, it is subject to various regulatory rules and regulations. As a result of these customer holdings, the Company is obligated by the U.S. Securities and Exchange Commission (“SEC”) and the Commodities 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 securities segregated under federal and other regulations approximate fair value.

Market making, sales, trading and execution activities

'

Market making, sales, trading and execution activities

Financial instruments owned and Financial instruments sold, not yet purchased, which relate to market making and trading activities, include listed and OTC equity securities, listed equity options and fixed income securities which are recorded on a trade date basis and carried at fair value. Trading revenue, net (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 FCM are recorded net of any commissions paid to independent brokers and are 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 clearing brokers, for facilitating the settlement and financing of securities transactions. Interest income and interest expense which have been netted within Interest, net on the Consolidated Statements of Operations are as follows (in thousands):

For the three months

For the nine months

ended September 30,

ended September 30,

2013

2012

2013

2012

Interest Income

$

3,992

$

1,170

$

6,022

$

1,847

Interest Expense

(4,169

)

(1,951

)

(6,992

)

(3,845

)

Interest, net

$

(177

)

$

(781

)

$

(970

)

$

(1,998

)

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

For the three months

For the nine months

ended September 30,

ended September 30,

2013

2012

2013

2012

Dividend Income

$

9,009

$

288

$

11,484

$

1,049

Dividend Expense

$

(6,899

)

$

(809

)

$

(8,035

)

$

(1,321

)

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

'

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 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, with additional collateral obtained or refunded 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.

Collateralized agreements related to Urban are recorded within Liabilities within discontinued operations on the Consolidated Statements of Financial Condition, and comprise:

•

Other secured financings, which represent contractual agreements used to finance financial instruments, are recorded at their contractual amount. These agreements are short-term in nature with durations of typically less than one month and bear interest at negotiated rates. Urban receives cash and pledges financial instruments to banks and other financial institutions as collateral for these secured financing arrangements. The market value of the collateral delivered must be in excess of the principal amount loaned plus the agreed upon margin requirement under the secured financings. The banks and other financial institutions may request additional collateral, if appropriate.

•

Liability to GNMA trusts, at fair value, represent the liability associated with Urban’s securitization of HECMs where the securitization does not meet the GAAP criteria for sale treatment.

The Company’s securities borrowed, securities loaned, financial instruments sold under agreements to repurchase and other secured financings 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 fully collateralized. These items would be categorized as Level 2 in the fair value hierarchy if they were required to be recorded at fair value.

Securitization activities

'

Securitization activities

Urban, which KCG has entered into a definitive agreement to sell as described in Footnote 1 "Organization and Description of the Business" and Footnote 4 "Discontinued Operations", securitizes home equity conversion mortgages (“HECMs”) under its GNMA issuance authority. Securitization and transfer of financial assets to a third party are generally accounted for as sales when an issuer has relinquished control over the transferred assets. Based upon the current structure of the GNMA securitization program, the Company believes that Urban has not met the GAAP criteria for relinquishing control over the transferred assets and therefore its securitizations fail to meet the GAAP criteria for sale accounting. As such, the Company continues to recognize the HECMs in Assets within discontinued operations, and the Company recognizes a corresponding liability to the GNMA trusts in Liabilities within discontinued operations on the Consolidated Statements of Financial Condition. The associated change in fair value of the securitized HECM loan inventory is recorded in Loss from discontinued operations, net of tax on the Consolidated Statements of Operations.

Investments

'

Investments

Investments primarily comprise strategic investments and deferred compensation investments. Strategic investments include noncontrolling equity ownership interests held by the Company within its non-broker-dealer subsidiaries, primarily in financial services-related businesses. Strategic investments are accounted for under the equity method or at cost. The equity method of accounting is used when the Company has significant influence, generally considered to be between 20% and 50% equity ownership in a corporation or greater than 3% to 5% of a partnership interest. Strategic investments are held at cost, less impairment if any, when the Company is not considered to exert significant influence on operating and financial policies of the investee.

Prior to the Mergers, the Company had a strategic investment in Knight which was classified as available for sale securities. Any gain or losses in the fair value of this investment were recorded in Other comprehensive income (loss).

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 amortizes other intangible assets 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.

Discontinued operations and Assets and Liabilities held for sale

'

Discontinued operations and Assets and Liabilities

Revenues and expenses associated with a business line that has been disposed of through closure or sale, or are considered held for sale, are included in Loss from discontinued operations, net of tax on the Consolidated Statements of Operations. Assets and liabilities of a disposal group classified as discontinued operations are included in Assets within discontinued operations and Liabilities within discontinued operations, respectively, on the Consolidated Statements of Financial Condition. Cash flows from discontinued operations are presented on the Consolidated Statements of Cash Flows within operating, investing and financing activities, as applicable.

Payable to customers

'

Payable to customers

Payable to customers arise primarily from futures transactions and include 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 or, in certain instances, as inducement grants to new hires.

Foreign currency translation and foreign currency forward contracts

'

Foreign currency translation and foreign currency forward contracts

The Company has foreign subsidiaries that do not utilize the U.S. dollar as their functional currency.

Assets and liabilities of the foreign subsidiaries having non-U.S. dollar functional currencies 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 (loss) income on the Consolidated Statements of Financial Condition and Cumulative translation adjustment on the Consolidated Statements of Comprehensive Income. Gains or losses resulting from foreign currency transactions are included in Investment income and other, net on the Company’s Consolidated Statements of Operations. For the three months ended September 30, 2013 and 2012, the Company recorded a loss of $0.1 million and a gain of $0.2 million, respectively. For the nine months ended September 30, 2013 and 2012, the Company recorded a loss of $1.3 million and a gain of $0.3 million, respectively.

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 (loss) income on the Consolidated Statements of Financial Condition and the Consolidated Statements of Comprehensive Income. The ineffective portion, if any, is recorded in Investment income and other, net on the Consolidated Statements of Operations.

Stock-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 RSUs and are eligible to retire prior to the vesting of such awards.

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 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 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 estimated sub-lease income. In the event the Company is able to sublease the excess real estate after recording a lease loss, 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. In the event that the Company concludes that previously determined excess real estate is needed for the Company’s use, such lease loss accrual is adjusted accordingly.

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

Variable interest entities

'

Variable interest entities

A VIE is an entity that lacks one or more of the following characteristics (i) the total equity investment at risk is sufficient to enable the entity to finance its activities independently and (ii) the equity holders have the power to direct the activities of the entity that most significantly impact its economic performance, the obligation to absorb the losses of the entity and the right to receive the residual returns of the entity. The Company has a controlling financial interest and will consolidate a VIE if it has both (i) the power to direct the activities of the VIE that most significantly impact the VIE’s economic performance and (ii) the obligation to absorb losses of the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE.

VIEs generally finance the purchase of assets by issuing debt and equity securities that are either collateralized by or indexed to the assets held by the VIE. The debt and equity securities issued by a VIE may include tranches of varying levels of subordination. The Company’s involvement with VIEs includes purchased interests and commitments to VIEs.

The Company is principally involved with VIEs through Urban’s securitization of HECM loan inventory.  Urban sells HECM loans to GNMA trusts, which have the characteristics of a VIE, and Urban retains certain commitments and obligations to the GNMA trusts.  Urban's maximum exposure to loss is the value of its obligations as issuer and servicer of the GNMA trusts. Securitized HECM loan inventory variable interests are recorded in Assets and liabilities within discontinued operations.

The following table presents the Company’s nonconsolidated VIEs at September 30, 2013 (in thousands):

30-Sep-13

Securitized

HECM loan

inventory recorded within Assets within discontinued operations

Carrying value of the variable interests

Assets

$

—

Liabilities

1,326

Maximum exposure to loss in nonconsolidated VIEs

Commitments

43,932

Purchased interests

—

The Company did not have any nonconsolidated VIEs at December 31, 2012

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 December 2011, the Financial Accounting Standards Board (“FASB”) issued an Accounting Standard Update (“ASU”) that requires additional disclosures about financial assets and liabilities that are subject to netting arrangements. Under the ASU, financial assets and liabilities must be disclosed at their respective gross asset and liability amounts, the amounts offset on the balance sheet and a description of the respective netting agreements. The new disclosures are required for reporting periods beginning on or after January 1, 2013, and are to be applied retrospectively. Other than requiring additional disclosures, the adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.

In February 2013, the FASB issued an ASU that requires additional disclosure requirements for items reclassified out of accumulated other comprehensive income. This new guidance requires entities to present either on the face of the income statement or in the notes to the financial statements the effects on the specific line items of the income statement for amounts reclassified out of accumulated other comprehensive income. This ASU is effective for reporting periods beginning after December 15, 2012. Other than additional disclosure requirements, the adoption of this ASU did not have an impact on the Company's Consolidated Financial Statements.

Merger_of_GETCO_and_Knight_Tab

Merger of GETCO and Knight (Tables)	9 Months Ended
	Sep. 30, 2013
Business Combinations [Abstract]	'
Schedule of Business Acquisitions, by Acquisition	'
	Amounts preliminarily allocated to intangible assets, the amortization period and goodwill were as follows (dollars in thousands):
							Amortization
			Amount				Years
	Technology		$	110,000			5 years
	Customer relationships		35,000				9 - 11 years
	Trade names		4,000				10 years
	Trading rights		7,800				7 years
	Intangible assets		156,800
	Goodwill		13,753
	Total		$	170,553
	The following table reflects the preliminary 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 securities 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				1,366,974
	Fixed assets and leasehold improvements, net				84,596
	Investments				106,353
	Intangible assets				156,800
	Assets within discontinued operations				5,607,063
	Other assets				211,735
	Total Assets				$	11,342,609
	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				139,624
	Liabilities within discontinued operations				5,518,168
	Long-term debt				375,000
	Total Liabilities				$	9,983,227
	Total identified assets acquired, net of assumed liabilities				1,359,382
	Goodwill				13,753
	Total Purchase Price				$	1,373,135
Business Acquisition, Pro Forma Information	'
	Included in KCG results for the three months ended September 30, 2013 are results from the businesses acquired as a result of the Mergers as follows:
		Three months ended September 30,
		2013
	Revenues	$	240,805
	Income from continuing operations, before income taxes	21,164
			Three months ended September 30, 2012							Nine months ended September 30, 2013					Nine months ended September 30, 2012
			Reported				Pro Forma			Reported		Pro Forma			Reported		Pro Forma
			(in thousands, except per share amounts)
	Revenue		$	130,602			$	(120,210	)	$	698,123	$	1,099,518		$	425,241	$	677,955
	Net income from continuing operations		9,807				(303,413		)	138,316		(10,494		)	24,556		(274,981		)
	Net income		9,807				(387,350		)	137,532		(39,554		)	24,556		(350,831		)
	Diluted earnings (loss) per share from continuing operations		$	0.21			$	(6.54	)	$	2.01	$	(0.15	)	$	0.49	$	(5.54	)
	Diluted earnings (loss) per share		$	0.21			$	(8.35	)	$	2	$	(0.57	)	$	0.49	$	(7.07	)
	The pro forma results are based on adding the pre-tax historical results of GETCO and Knight, and adjusting primarily for amortization of intangibles created in the Mergers, debt raised in conjunction with the Mergers and income taxes as if the Company was subject to U.S. corporate income taxes for all periods presented. The pro forma data assumes all GETCO units have been converted to KCG Class A Common Stock on January 1, 2012 and excludes any gain recognized on Knight Common Stock. The pro forma disclosures do not include adjustments to reflect the Company's operating costs or expected differences in the way funds generated by the Company are invested. The pro forma data is intended for informational purposes and is not indicative of the future results of operations.

Significant_Accounting_Policie2

Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]	'
Schedule of Interest Income and Interest Expense	'
	Interest income and interest expense which have been netted within Interest, net on the Consolidated Statements of Operations are as follows (in thousands):
		For the three months						For the nine months
		ended September 30,						ended September 30,
		2013			2012			2013			2012
	Interest Income	$	3,992		$	1,170		$	6,022		$	1,847
	Interest Expense	(4,169		)	(1,951		)	(6,992		)	(3,845		)
	Interest, net	$	(177	)	$	(781	)	$	(970	)	$	(1,998	)
Net Trading Revenue Including Dividend Income and Expense	'
	Trading revenue, net includes dividend income and expense as follows (in thousands):
		For the three months						For the nine months
		ended September 30,						ended September 30,
		2013			2012			2013			2012
	Dividend Income	$	9,009		$	288		$	11,484		$	1,049
	Dividend Expense	$	(6,899	)	$	(809	)	$	(8,035	)	$	(1,321	)
Company's Nonconsolidated VIEs	'
	The following table presents the Company’s nonconsolidated VIEs at September 30, 2013 (in thousands):
		30-Sep-13
		Securitized
		HECM loan
		inventory recorded within Assets within discontinued operations
	Carrying value of the variable interests
	Assets	$	—
	Liabilities	1,326
	Maximum exposure to loss in nonconsolidated VIEs
	Commitments	43,932
	Purchased interests	—
	The Company did not have any nonconsolidated VIEs at December 31, 2012.

Discontinued_Operations_Tables

Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2013
Discontinued Operations and Disposal Groups [Abstract]	'
Summary of Revenues and Results of Operations of Discontinued Operations	'
	The revenues and results of operations of discontinued operations are summarized as follows (in thousands):
		For the three and nine months
		ended September 30,
		2013
	Revenues	$	22,114
	Expenses:
	Compensation	$	8,379
	Execution and clearance fees	2,840
	Payments for order flow	5,286
	Other expenses	6,873
	Total Expenses	23,378
	Pre-tax loss from discontinued operations	(1,264		)
	Income tax benefit	480
	Loss from discontinued operations, net of tax	$	(784	)
	Assets and liabilities within discontinued operations are presented in the following table (in thousands):
		September 30,
		2013
	Assets within discontinued operations:
	Cash and cash equivalents	$	15,702
	Securitized HECM loan inventory	5,841,664
	Loan inventory	177,674
	Receivable from brokers, dealers and clearing organizations	37,411
	Other assets	25,848
	Total assets	$	6,098,299
	Liabilities within discontinued operations:
	Liability to GNMA trusts	5,803,859
	Other secured financings	129,552
	Payable to brokers, dealers and clearing organizations	8,538
	Accrued compensation expense	4,717
	Accrued expenses and other liabilities	59,358
	Total liabilities	$	6,006,024

Assets_Segregated_or_Held_in_S1

Assets Segregated or Held in Seperate Accounts Under Federal or Other Regulations (Tables)	9 Months Ended
	Sep. 30, 2013
Cash and Cash Equivalents [Abstract]	'
Cash and Securities Segregated Under U.S. Federal and Other Regulation	'
	Cash and securities segregated under U.S. federal and other regulations primarily relate to the Company’s FCM business and consist of the following (in thousands):
		September 30,
		2013
	Cash and cash equivalents segregated under federal or other regulations	$	216,442
	Receivables from brokers, dealers and clearing organizations	282,650
	Total assets segregated or held in separate accounts under federal or other regulations	$	499,092

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
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
	30-Sep-13	Level 1				Level 2				Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	2,156,466			$	—			$	—		$	2,156,466
	Listed options	288,227				—				—			288,227
	U.S. government and Non-U.S. government obligations	32,678				—				—			32,678
	Corporate debt	46,606				—				—			46,606
	Total Financial instruments owned, at fair value	2,523,977				—				—			2,523,977
	Securities on deposit with clearing organizations (2)	160,369				—				—			160,369
	Deferred compensation investments (3)	—				84				—			84
	Investment in Deephaven Funds (3)	—				1,430				—			1,430
	Assets within discontinued operations:												—
	Securitized HECM loan inventory (4)	—				5,841,664				—			5,841,664
	Loan inventory	—				177,674				—			177,674
	Total fair value of financial instrument assets	$	2,684,346			$	6,020,852			$	—		$	8,705,198
	Liabilities
	Financial instruments sold, not yet purchased, at fair value:
	Equities (1)	$	1,848,729			$	—			$	—		$	1,848,729
	U.S. government obligations	29,441				—				—			29,441
	Corporate debt	49,616				—				—			49,616
	Listed options	229,721				—				—			229,721
	Foreign currency forward contracts	—				5,431				—			5,431
	Total Financial instruments sold, not yet purchased, at fair value	2,157,507				5,431				—			2,162,938
	Liabilities within discontinued operations:
	Liability to GNMA trusts, at fair value (4)	—				5,803,859				—			5,803,859
	Total fair value of financial instrument liabilities	$	2,157,507			$	5,809,290			$	—		$	7,966,797
	________________________________________
	(1)	Equities of $1.22 billion have been netted by their respective CUSIP number and their long and short positions.
	(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)	Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition.
	(4)	Represents HECMs that have been securitized into HECM Mortgage Backed Securities (“HMBS”) where the securitization is not accounted for as a sale of the underlying HECMs. See Securitized HECM loan inventory below for full description.
		Assets and Liabilities Measured at
		Fair Value on a Recurring Basis
	31-Dec-12	Level 1				Level 2				Level 3			Total
	Assets
	Financial instruments owned, at fair value:
	Equities (1)	$	378,933			$	—			$	—		$	378,933
	Listed options	92,305				—				—			92,305
	U.S. government obligations and corporate bonds	68,765				—				—			68,765
	Mutual funds - Bond Funds	114,872								—			114,872
	Total Financial instruments owned, at fair value	654,875				—				—			654,875
	Securities on deposit with clearing organizations	7,147								—			7,147
	Investment - Knight preferred shares	199,632								—			199,632
	Total fair value of financial instrument assets	$	861,654			$	—			$	—		$	861,654
	Liabilities
	Financial instruments sold, not yet purchased, at fair value:
	Equities (1)	$	423,740			$	—			$	—		$	423,740
	U.S. government obligations	19,056				—				—			19,056
	Listed options	69,757				—				—			69,757
	Total Financial instruments sold, not yet purchased, at fair value	512,553				—				—			512,553
	Total fair value of financial instrument liabilities	$	512,553			$	—			$	—		$	512,553
	___________________________________
	(1) Equities of $5.9 million have been netted by their respective CUSIP number and their long and short positions.
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 of derivative instruments and futures contract trading activities in the Consolidated Statements of Financial Condition and the gains and losses included in the Consolidated Statements of Operations (fair value in thousands):
				30-Sep-13
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		$	21			651			$	62		244
	Forward contracts (1)	Financial instruments owned, at fair value		—				—			5,431			2
	Forward contracts (1)	Receivables from/Payables to brokers, dealers and clearing organizations		331				1			—			—
	Equity
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		1,442				3,059			595			1,046
	Swap contracts	Receivables from/Payables to brokers, dealers and clearing organizations		230				1			—			—
	Listed options	Financial instruments owned, at fair value		288,227				772,768			229,721			753,979
	Fixed income
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		4,183				24,400			1,724			6,601
	Commodity
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		732				2,278			1,478			3,565
	Total			$	295,166			803,158			$	239,011		765,437
	(1) Designated as hedging instrument
				31-Dec-12
		Financial Statements		Assets							Liabilities
		Location		Fair Value				Contracts			Fair Value			Contracts
	Foreign currency
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		$	8			500			$	—		95
	Forward contracts	Accrued expenses and other liabilities		—				—			110			1
	Equity
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		498				1,027			70			234
	Swap contracts	Receivables from/Payables to brokers, dealers and clearing organizations		570				1			—			—
	Listed options	Financial instruments owned, at fair value		92,305				199,324			69,757			196,804
	Fixed income
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		3,474				42,143			5,043			61,398
	Commodity
	Futures contracts	Receivables from/Payables to brokers, dealers and clearing organizations		428				1,385			410			1,824
	Total			$	97,283			244,380			$	75,390		260,356
Fair Value of Derivative Instruments Gain Loss Recognized	'
					Gain (Loss) Recognized
			Financial Statements		For the three months
					ended September 30,
			Location		2013				2012
	Derivative instruments not designated as hedging instruments:
	Foreign currency
	Futures contracts		Trading revenues, net		$	3,825			$	4,493
	Equity
	Futures contracts		Trading revenues, net		17,461				10,724
	Swap contracts		Trading revenues, net		1,884				1,985
	Listed options (1)		Trading revenues, net		17,044				24,582
	Fixed income
	Futures contracts		Trading revenues, net		22,008				15,557
	Commodity
	Futures contracts		Trading revenues, net		18,774				8,792
					$	80,996			$	66,133
	Derivative instruments designated as hedging instruments:
	Foreign exchange - forward contract		Accumulated other comprehensive (loss)		$	(4,948	)		$	—
	(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 revenue, net on the Company’s Consolidated Statements of Operations.
					Gain (Loss) Recognized
			Financial Statements		For the nine months ended September 30,
			Location		2013				2012
	Derivative instruments not designated as hedging instruments:
	Foreign currency
	Futures contracts		Trading revenues, net		$	9,243			$	16,446
	Equity
	Futures contracts		Trading revenues, net		44,125				62,241
	Swap contracts		Trading revenues, net		10,755				5,007
	Listed options (1)		Trading revenues, net		63,322				41,455
	Fixed income
	Futures contracts		Trading revenues, net		64,211				66,573
	Commodity
	Futures contracts		Trading revenues, net		44,053				26,065
					$	235,709			$	217,787
	Derivative instruments designated as hedging instruments:
	Foreign exchange - forward contract		Accumulated other comprehensive (loss)		$	(4,948	)		$	—
	(1)	Realized gains and losses on listed 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 revenue, net on the Company’s Consolidated Statements of Operations.
Gross amounts of recognized derivative assets and gross amounts of offsets in the Consolidated Statement of Condition	'
	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):
		30-Sep-13	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts of Assets Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
														Financial Instruments		Cash Collateral Received
		Assets
		Listed options	$	288,227			$	—			$	288,227		$	—	$	—	$	288,227
		Securities borrowed	1,370,921				—				1,370,921			1,329,508		—		41,413
		Total Assets	$	1,659,148			$	—			$	1,659,148		$	1,329,508	$	—	$	329,640
		Liabilities
		Listed options	$	229,722			$	—			$	229,722		$	—	$	5,064	$	224,658
		Securities loaned	543,451				—				543,451			533,266		—		10,185
		Financial instruments sold under agreements to repurchase	595,029				—				595,029			595,022		—		7
		Foreign currency forward contracts	5,431				—				5,431			—		—		5,431
		Total Liabilities	$	1,373,633			$	—			$	1,373,633		$	1,128,288	$	5,064	$	240,281
		31-Dec-12	Gross Amounts Recognized				Gross Amounts Offset in the Statements of Financial Condition				Net Amounts of Assets Presented in the Statements of Financial Condition			Gross Amounts Not Offset in the Statement of Financial Condition				Net Amount
														Financial Instruments		Cash Collateral Received
		Assets
		Listed options	$	92,305			$	—			$	92,305		$	—	$	—	$	92,305
		Securities borrowed	52,261				—				52,261			50,717				1,544
		Total Assets	$	144,566			$	—			$	144,566		$	50,717	$	—	$	93,849
		Liabilities
		Listed options	$	69,757			$	—			$	69,757		$	—	$	1,393	$	68,364
		Total Liabilities	$	69,757			$	—			$	69,757		$	—	$	1,393	$	68,364

Collateralized_Transactions_Ta

Collateralized Transactions (Tables)	9 Months Ended
	Sep. 30, 2013
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 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 counterparty (in thousands):
		September 30,		December 31,
		2013		2012
	Collateral permitted to be delivered or repledged	$	1,317,369	$	50,717
	Collateral that was delivered or repledged	1,210,408		50,717
	Collateral permitted to be further repledged by the receiving counterparty	149,891		—
Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge	'
	The table below presents information about assets pledged by the Company (in thousands):
		September 30,		December 31,
		2013		2012
	Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge (1)	$	6,803,143	$	52,261
	Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	732,481		—
	(1)	Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge includes $5.84 billion recorded within Assets within discontinued operations as of September 30, 2013.

Receivable_from_and_Payable_to1

Receivable from and Payable to Brokers, Dealers and Clearing Organizations (Tables)	9 Months Ended
	Sep. 30, 2013
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):
		September 30,		31-Dec-12
		2013
	Receivable:
	Clearing organizations and other	$	1,096,308	$	140,089
	Securities failed to deliver	233,805		2,880
	Total Receivable	$	1,330,113	$	142,969
	Payable:
	Clearing organizations and other	$	564,592	$	15,879
	Securities failed to receive	101,586		8,306
	Total Payable	$	666,178	$	24,185

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2013
Investments [Abstract]	'
Summary of Investments	'
	Investments comprise strategic investments, including limited partnership investments, deferred compensation investments related to employee and director deferred compensation plans and investment in the Deephaven Funds. Investments consist of the following (in thousands):
		September 30,		December 31,
		2013		2012
	Strategic investments:
	Investments accounted for under the equity method	$	82,476	$	—
	Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	41,899		48,806
	Total Strategic investments	124,375		48,806
	Deferred compensation investments	84		—
	Knight preferred shares	—		199,632
	Investment in Deephaven Funds	1,430		—
	Total Investments	$	125,889	$	248,438

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2013
Goodwill and Intangible Assets Disclosure [Abstract]	'
Summary of Goodwill	'
	The following table summarizes the Company’s Goodwill by segment (in thousands):
		September 30,				31-Dec-12
		2013
	Market Making	$	15,159			$	4,645
	Global Execution Services	3,239				—
	Total	$	18,398			$	4,645
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):
		September 30,				31-Dec-12
		2013
	Market Making
	Trading rights	$	47,065			$	42,635
	Technology	52,791				3,488
	Total	99,856				46,123
	Global Execution Services
	Customer relationships	34,139				—
	Trade names	3,900				—
	Technology	39,900				—
	Total	77,939				—
	Corporate and Other
	Technology	14,250
	Consolidated Total	$	192,045			$	46,123
			September 30,				31-Dec-12
			2013
	Customer relationships (1)	Gross carrying amount	$	35,000			$	—
		Accumulated amortization	(861		)		—
		Net carrying amount	34,139				—
	Trading rights (2)	Gross carrying amount	59,628				51,828
		Accumulated amortization	(12,563		)		(9,193		)
		Net carrying amount	47,065				42,635
	Trade names (3)	Gross carrying amount	4,000				—
		Accumulated amortization	(100		)		—
		Net carrying amount	3,900				—
	Technology (4)	Gross carrying amount	115,580				5,580
		Accumulated amortization	(8,639		)		(2,092		)
		Net carrying amount	106,941				3,488
	Total	Gross carrying amount	214,208				57,408
		Accumulated amortization	(22,163		)		(11,285		)
		Net carrying amount	$	192,045			$	46,123
	________________________________________
	(1)	Customer relationships relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of September 30, 2013. Lives may be reduced depending upon actual retention rates.
	(2)	Trading rights provide the Company with the rights to trade on certain exchanges. The weighted average remaining life is approximately 9 and 10 years as of September 30, 2013 and December 31, 2012, respectively.
	(3)	Trade names relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of September 30, 2013.
	(4)	The weighted average remaining life is approximately five and three years as of September 30, 2013 and December 31, 2012, respectively.
Summary of Amortization Expense Relating to Intangible Assets	'
	The following table summarizes the Company’s amortization expense from continuing operations relating to Intangible assets (in thousands):
		For the three months								For the nine months
		ended September 30,								ended September 30,
		2013				2012				2013		2012
	Amortization expense	$	8,119			$	1,380			$	10,878	$	4,139
Summary of Estimated Amortization Expense for Future Years	'
	As of September 30, 2013, the following table summarizes the Company’s estimated amortization expense for future periods (in thousands):
		Amortization
		expense
	For the three months ending December 31, 2013	$	8,119
	For the year ended December 31, 2014	32,477
	For the year ended December 31, 2015	31,780
	For the year ended December 31, 2016	31,082
	For the year ended December 31, 2017	31,082

LongTerm_Debt_Tables

Long-Term Debt (Tables)	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Summary of Long-Term Debt	'
	The carrying value and fair value of the Company's Long-term debt is as follows (in thousands):
		30-Sep-13				31-Dec-12
		Carrying Amount		Fair Value		Carrying Amount		Fair Value
	Notes	$	—	$	—	$	15,000	$	15,000
	Senior Secured Notes	305,000		299,663		—		—
	Credit Agreement	535,000		535,000		—		—
	Convertible Notes	117,259		117,845		—		—
	Total	957,259		952,508		15,000		15,000
	Less: Current portion	235,000		235,000		—		—
	Total Long-term debt	$	722,259	$	717,508	$	15,000	$	15,000
Recorded Expenses with Respect to Long-Term Debt	'
	The Company recorded expenses with respect to the Long term debt as follows (in thousands):
		For the three months				For the nine months
		ended September 30,				ended September 30,
		2013		2012		2013		2012
	Interest expense	$	19,173	$	688	$	21,366	$	1,134
	Amortization of debt issuance cost (1)	2,007		103		2,479		152
	Commitment fee (1)	267		390		9,204		390
	Total	$	21,447	$	1,181	$	33,049	$	1,676
	(1)	Included in Other expense.

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2013
Share-based Compensation [Abstract]	'
Compensation Expense Relating to Restricted Awards	'
	The following is a schedule of the changes in the Company’s unvested Class E units (units in thousands):
			Units			Weighted Average Grant Price
	Unvested as of December 31, 2012		476			$	36.78
	Issued		—			—
	Vested		(442	)		37.24
	Forfeited		(34	)		30.89
	Unvested as of September 30, 2013		—			$	—
	The following is a schedule of the changes in the Company’s unvested Class B units (units in thousands):
			Units			Weighted Average Grant Price
	Unvested as of December 31, 2012		417			$	89.99
	Issued		58			73.54
	Vested		(438	)		87.86
	Forfeited		(37	)		89.28
	Unvested as of September 30, 2013		—			$	—
	The following is a schedule of the changes in the Company’s unvested Class E units (units in thousands):
			Units			Weighted Average Grant Price
	Unvested as of December 31, 2012		476			$	36.78
	Issued		—			—
	Vested		(442	)		37.24
	Forfeited		(34	)		30.89
	Unvested as of September 30, 2013		—			$	—
	The following is a summary of the changes in the incentive units for the nine months ended September 30, 2013 (units in thousands):
		Vested			Unvested
	Incentive units at January 1, 2013	24			45
	Issued	1			12
	Vested	53			(53	)
	Exercised	(22	)		—
	Canceled	(1	)		(4	)
	Incentive units at September 30, 2013	55			—
	Compensation expense from continuing operations relating to restricted awards, primarily recorded in Employee compensation and benefits, and the corresponding income tax benefit, which was recorded in Income tax benefit on the Consolidated Statements of Operations are presented in the following table (in thousands):
		For the three months
		ended September 30,
		2013
	Stock award compensation expense	$	12,234
	Income tax benefit	4,649
Summary of Restricted Awards Activity	'
	The following table summarizes restricted awards activity, including awards related to employees working in businesses that are included within discontinued operations, for the three months ended September 30, 2013 (awards in thousands):
			Restricted Stock Units
			Number of			Weighted-
			Units			Average
						Grant date
						Fair Value
	1-Jul-13		—			$	—
	Conversion of outstanding Knight RSUs		3,251			11.22
	Granted		6,004			10.58
	Vested		(45	)		11.07
	Forfeited		(246	)		11.45
	Outstanding at September 30, 2013		8,963			$	10.78
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 on the Consolidated Statements of Operations are as follows (in thousands):
		For the three months
		ended September 30,
		2013
	Stock option and SAR compensation expense	$	915
	Income tax benefit	348
Summary of Stock Option Activity	'
	The following table summarizes stock option and SAR activity and stock options exercisable for the three months ended September 30, 2013 (awards in thousands):
		Number of Stock Awards			Weighted-				Aggregate				Weighted-
					Average				Intrinsic				Average
					Exercise				Value				Remaining
					Price								Life (years)
	1-Jul-13	—			$	—
	Conversion of outstanding Knight options	782			41.25
	Granted at market value (1)	4,324			15.19
	Exercised	—			—
	Forfeited or expired	(79	)		45.81
	Outstanding at September 30, 2013 (1)	5,027			$	18.76			$	387			4.49
	Exercisable at September 30, 2013	703			$	40.74			$	381			2.61
	Available for future grants at September 30, 2013 *	6,327
	* Represents both options and awards available for grant.
	(1) Includes 1.7 million of SARs.
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):
			Three months ended September 30,								Nine months ended September 30,
			2013				2012				2013			2012
	Class B and E units		$	—			$	5,429			$	19,860		$	9,124
	Incentive units		(875		)		368				1,428			788
	Total		$	(875	)		$	5,797			$	21,288		$	9,912

Income_Taxes_Tables

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
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 three months						For the nine months
		ended September 30,						ended September 30,
		2013			2012			2013			2012
	U.S. federal statutory income tax (benefit) expense	$	41,949		$	5,114		$	12,543		$	12,224
	Income not subject to U.S. corporate income tax	(44,790		)	(3,515		)	(14,071		)	(9,801		)
	U.S. state and local income taxes, net of U.S. federal income tax effect	(391		)	533			82			825
	Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	(103,499		)	—			(103,499		)	—
	Nondeductible charges (1)	(1,114		)	74			(944		)	166
	Foreign taxes	—			2,581			3,559			6,958
	Other, net	78			18			(148		)	(4		)
	Income tax (benefit) expense	$	(107,767	)	$	4,805		$	(102,478	)	$	10,368

Accumulated_Other_Comprehensiv1

Accumulated Other Comprehensive Loss (Tables)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Accumulated Other Comprehensive Income (Loss)	'
	The following table presents changes in Accumulated other comprehensive loss, net of tax by component for the nine months ended September 30, 2013 (in thousands):
		Unrealized Gains (Losses) on Available-for-Sale Securities			Foreign Currency Translation Adjustments			Total
	Balance January 1, 2013	$	114,319		$	—		$	114,319
	Other comprehensive income	13,604			(5,098		)	8,506
	Reclassification of gain on investment in Knight Common Stock	(127,972		)	—			(127,972		)
	Net current-period other comprehensive loss	(114,368		)	(5,098		)	(119,466		)
	Balance September 30, 2013	$	(49	)	$	(5,098	)	$	(5,147	)

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Schedule of Reconcililation of Earnings Per Share	'
	The following is a reconciliation of the numerators and denominators of the basic and diluted earnings per share computations from continuing operations for the three and nine months ended September 30, 2013 and 2012 (in thousands):
		For the three months ended September 30,
		2013				2012
		Numerator /		Denominator /		Numerator /		Denominator /
						net
		net income		shares		income		shares
	Income from continuing operations and shares used in basic calculations	$	227,621	114,113		$	9,807	46,411
	Effect of dilutive stock based awards			660				—
	Income from continuing operations and shares used in diluted calculations	$	227,621	114,773		$	9,807	46,411
	Basic earnings per share from continuing operations			$	1.99			$	0.21
	Diluted earnings per share from continuing operations			$	1.98			$	0.21
		For the nine months ended September 30,
		2013				2012
		Numerator /		Denominator /		Numerator /		Denominator /
						net
		net income		shares		income		shares
	Income from continuing operations and shares used in basic calculations	$	138,316	68,632		$	24,556	49,619
	Effect of dilutive stock based awards			223				—
	Income from continuing operations and shares used in diluted calculations	$	138,316	68,855		$	24,556	49,619
	Basic earnings per share from continuing operations			$	2.02			$	0.49
	Diluted earnings per share from continuing operations			$	2.01			$	0.49

Commitments_and_Contingent_Lia1

Commitments and Contingent Liabilities (Tables)	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Schedule of Lease and Contract Obligations	'
	The future minimum payments including interest under the capitalized leases at September 30, 2013 consist of (in thousands):
			Minimum Payments
	Through December 31, 2013		$	2,885
	2014		8,222
	2015		2,072
	Total		$	13,179
Schedule of Interest Expense, Capital Lease	'
	The total interest expense related to capital leases for the three and nine months ended September 30, 2013 and 2012 included in the Consolidated Statements Operations is as follows (in thousands):
		Three months ended September 30,					Nine months ended September 30,
		2013			2012		2013		2012
	Interest expense - Capital leases	$	151		$	285	$	602	$	1,135
Schedule of Future Minimum Rental Payments for Operating Leases	'
	The Company leases certain computer and other equipment under noncancelable operating leases. As of September 30, 2013, 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
	Three months ending December 31, 2013	$	8,014		$	525	$	7,489
	Year ending December 31, 2014	30,170			1,868		28,302
	Year ending December 31, 2015	28,346			1,494		26,852
	Year ending December 31, 2016	27,877			1,464		26,413
	Year ending December 31, 2017	27,262			1,541		25,721
	Thereafter through December 31, 2027	108,318			358		107,960
	Total	$	229,987		$	7,250	$	222,737

Business_Segments_Tables

Business Segments (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Income (Loss) from Continuing Operations Before Income Taxes and Total Assets by Segment	'
	The Company’s revenues, income (loss) from continuing operations before income taxes (“Pre-tax earnings”) and total assets by segment are summarized in the following table (in thousands):
		Market		Global Execution Services			Corporate		Consolidated
		Making					and Other		Total
	For the three months ended September 30, 2013:
	Revenues	$	240,110	$	91,366		$	136,343	$	467,819
	Pre-tax earnings	47,853		(16,354		)	88,355		119,854
	Total assets	3,882,763		1,426,824			1,885,408		7,194,995
	For the three months ended September 30, 2012:
	Revenues	$	107,672	$	9,258		$	13,672	$	130,602
	Pre-tax earnings	8,214		(1,120		)	7,518		14,612
	Total assets	1,507,881		25,934			239,490		1,773,305
	For the nine months ended September 30, 2013:
	Revenues	$	455,678	$	113,701		$	128,744	$	698,123
	Pre-tax earnings	55,608		(21,248		)	1,478		35,838
	Total assets	3,882,763		1,426,824			1,885,408		7,194,995
	For the nine months ended September 30, 2012:
	Revenues	$	383,203	$	27,248		$	14,790	$	425,241
	Pre-tax earnings	37,976		(3,839		)	787		34,924
	Total assets	1,507,881		25,934			239,490		1,773,305

Merger_of_GETCO_and_Knight_Det

Merger of GETCO and Knight (Detail) (USD $)	3 Months Ended	9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Jul. 02, 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	'
Cash paid for acquisition	'	'	$720
Shares granted to former shareholders (in shares)	'	41,900,000	'
Jefferies [Member]	'	'	'
Merger Transaction [Line Items]	'	'	'
Maximum cash entitled by shareholders to receive under the agreement	'	720	'
GETCO [Member]	'	'	'
Merger Transaction [Line Items]	'	'	'
Number of shares to be received by GETCO members	'	75,900,000	'
Number of warrants to acquire shares of common stock	24,300,000	24,300,000	'
Shares outstanding (in shares)	'	'	116,800,000
Warrants one [Member]	'	'	'
Merger Transaction [Line Items]	'	'	'
Warrants receivable from holding company	'	8,100,000	'
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	'	8,100,000	'
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	'	8,100,000	'
Exercise price of warrant (in dollars per share)	$15	$15	'
Term of warrant	'	'6 years	'
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	'
Knight [Member]	'	'	'
Merger Transaction [Line Items]	'	'	'
Deferred tax asset	'	'	65.9
Merger costs	1.2	27.6	'
Gain recognized	$128	$128	'

Merger_of_GETCO_and_Knight_Ass

Merger of GETCO and Knight - Assets Acquired and Liabilties Assumed (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Jul. 02, 2013
In Thousands, unless otherwise specified			Knight [Member]
Identifiable Net Assets	'	'	'
Cash and cash equivalents	'	'	$509,133
Cash and securities 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	'	'	1,366,974
Fixed assets and leasehold improvements, net	'	'	84,596
Investments	'	'	106,353
Intangible assets	'	'	156,800
Assets within discontinued operations	'	'	5,607,063
Other assets	'	'	211,735
Total Assets	'	'	11,342,609
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract]	'	'	'
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	'	'	139,624
Liabilities within discontinued operations	'	'	5,518,168
Long-term debt	'	'	375,000
Total Liabilities	'	'	9,983,227
Total identified assets acquired, net of assumed liabilities	'	'	1,359,382
Goodwill	18,398	4,645	13,753
Total Purchase Price	'	'	$1,373,135

Merger_of_GETCO_and_Knight_Int

Merger of GETCO and Knight - Intangible Assets Acquired (Details) (USD $)

9 Months Ended

12 Months Ended

1 Months Ended

1 Months Ended

1 Months Ended

1 Months Ended

In Thousands, unless otherwise specified

Sep. 30, 2013

Dec. 31, 2012

Jul. 02, 2013

Jul. 31, 2013

Jul. 02, 2013

Jul. 02, 2013

Jul. 31, 2013

Jul. 02, 2013

Jul. 31, 2013

Jul. 02, 2013

Jul. 31, 2013

Jul. 31, 2013

Knight [Member]

Technology [Member]

Technology [Member]

Customer relationships [Member]

Trade names [Member]

Trade names [Member]

Trading rights [Member]

Trading rights [Member]

Minimum [Member]

Maximum [Member]

Knight [Member]

Knight [Member]

Knight [Member]

Knight [Member]

Knight [Member]

Knight [Member]

Knight [Member]

Customer relationships [Member]

Customer relationships [Member]

Knight [Member]

Knight [Member]

Business Acquisition [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Amount

'

'

$156,800

'

$110,000

$35,000

'

$4,000

'

$7,800

'

'

Goodwill

18,398

4,645

13,753

'

'

'

'

'

'

'

'

'

Total

'

'

$170,553

'

'

'

'

'

'

'

'

'

Weighted average useful life

'7 years

'10 years

'

'5 years

'

'

'10 years

'

'7 years

'

'9 years

'11 years

Merger_of_GETCO_and_Knight_Pro

Merger of GETCO and Knight - Proforma Information (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Business Acquisition [Line Items]	'	'	'	'
Revenues - Reported	$467,819	$130,602	$698,123	$425,241
Net income from continuing operations- Reported	119,854	14,612	35,838	34,924
Net income	226,837	9,807	137,532	24,556
Diluted earnings per share from continuing operations (in dollars per share)	$1.98	$0.21	$2.01	$0.49
Diluted earnings per share (in dollars per share)	$1.98	$0.21	$2	$0.49
Knight [Member]	'	'	'	'
Business Acquisition [Line Items]	'	'	'	'
Revenues	240,805	'	'	'
Income from continuing operations, before income taxes	21,164	'	'	'
Revenues - Reported	130,602	'	698,123	425,241
Revenues - Pro Forma	-120,210	'	1,099,518	677,955
Net income from continuing operations- Reported	9,807	'	138,316	24,556
Net income from continuing operations - Pro Forma	-303,413	'	-10,494	-274,981
Net income	9,807	'	137,532	24,556
Net income - Pro Forma	($387,350)	'	($39,554)	($350,831)
Diluted earnings per share from continuing operations (in dollars per share)	$0.21	'	$2.01	$0.49
Diluted earnings per share from continuing operations - Pro Forma	($6.54)	'	($0.15)	($5.54)
Diluted earnings per share (in dollars per share)	$0.21	'	$2	$0.49
Diluted earnings per share - Pro Forma	($8.35)	'	($0.57)	($7.07)

Significant_Accounting_Policie3

Significant Accounting Policies - Additional Information (Detail) (USD $)	9 Months Ended		3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013
			Foreign currency transactions [Member]	Foreign currency transactions [Member]	Foreign currency transactions [Member]	Foreign currency transactions [Member]	Software [Member]	Minimum [Member]	Maximum [Member]
Property, Plant and Equipment [Line Items]	'	'	'	'	'	'	'	'	'
Maximum maturity of short-term investments	'90 days	'	'	'	'	'	'	'	'
Equity ownership	20.00%	20.00%	'	'	'	'	'	20.00%	50.00%
Partnership interest	'	'	'	'	'	'	'	3.00%	5.00%
Investment gain (loss) and other, net	'	'	($0.10)	$0.20	($1.30)	$0.30	'	'	'
Fixed assets, useful life	'	'	'	'	'	'	'	'3 years	'7 years
Amortization period, in years	'	'	'	'	'	'	'3 years	'4 years	'12 years

Significant_Accounting_Policie4

Significant Accounting Policies - Schedule of Interest Income and Interest Expense (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accounting Policies [Abstract]	'	'	'	'
Interest Income	$3,992	$1,170	$6,022	$1,847
Interest Expense	-4,169	-1,951	-6,992	-3,845
Interest, net	($177)	($781)	($970)	($1,998)

Significant_Accounting_Policie5

Significant Accounting Policies - Net Trading Revenue Including Dividend Income and Expense (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accounting Policies [Abstract]	'	'	'	'
Dividend Income	$9,009	$288	$11,484	$1,049
Dividend Expense	($6,899)	($809)	($8,035)	($1,321)

Significant_Accounting_Policie6

Significant Accounting Policies - Company's Nonconsolidated VIEs (Detail) (Securitized HECM loan inventory [Member], USD $)	Sep. 30, 2013
In Thousands, unless otherwise specified
Securitized HECM loan inventory [Member]	'
Carrying value of the variable interests	'
Assets	$0
Liabilities	1,326
Maximum exposure to loss in nonconsolidated VIEs	'
Commitments	43,932
Purchased interests	$0

Discontinued_Operations_Income

Discontinued Operations - Income Disclosures (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Expenses:	'	'	'	'
Compensation	$129,631	$31,875	$236,983	$111,395
Payments for order flow	16,431	717	17,468	2,128
Other	11,318	5,349	30,028	19,002
Total expenses	347,965	115,990	662,285	390,317
Loss from discontinued operations, net of tax	-784	0	-784	0
Segment, Discontinued Operations [Member]	'	'	'	'
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'	'	'
Revenues	22,114	'	22,114	'
Expenses:	'	'	'	'
Compensation	8,379	'	8,379	'
Execution and clearance fees	2,840	'	2,840	'
Payments for order flow	5,286	'	5,286	'
Other	6,873	'	6,873	'
Total expenses	23,378	'	23,378	'
Pre-tax loss from discontinued operations	-1,264	'	-1,264	'
Income tax benefit	480	'	480	'
Loss from discontinued operations, net of tax	($784)	'	($784)	'

Discontinued_Operations_Balanc

Discontinued Operations - Balance Sheet Disclosures (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Assets	'	'	'	'
Cash and cash equivalents	$798,712	$427,631	$423,508	$607,689
Other assets	287,015	23,073	'	'
Total assets	13,293,295	1,687,536	'	'
Liabilities within discontinued operations:	'	'	'	'
Payable to brokers, dealers and clearing organizations	666,178	24,185	'	'
Accrued expenses and other liabilities	220,648	118,068	'	'
Total liabilities	11,780,274	721,725	'	'
Segment, Discontinued Operations [Member]	'	'	'	'
Assets	'	'	'	'
Cash and cash equivalents	15,702	'	'	'
Securitized HECM loan inventory	5,841,664	'	'	'
Loan inventory	177,674	'	'	'
Receivable from brokers, dealers and clearing organizations	37,411	'	'	'
Other assets	25,848	'	'	'
Total assets	6,098,299	'	'	'
Liabilities within discontinued operations:	'	'	'	'
Liability to GNMA trusts	5,803,859	'	'	'
Other secured financings	129,552	'	'	'
Payable to brokers, dealers and clearing organizations	8,538	'	'	'
Accrued compensation expense	4,717	'	'	'
Accrued expenses and other liabilities	59,358	'	'	'
Total liabilities	$6,006,024	'	'	'

Assets_Segregated_or_Held_in_S2

Assets Segregated or Held in Separate Accounts Under Federal or Other Regulations - (Details) (USD $)	Sep. 30, 2013
In Thousands, unless otherwise specified
Cash and Cash Equivalents [Abstract]	'
Cash and cash equivalents segregated under federal or other regulations	$216,442
Receivables from brokers, dealers and clearing organizations	282,650
Total assets segregated or held in separate accounts under federal or other regulations	$499,092

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	$2,523,977		$654,875
Total fair value of financial instrument assets	8,705,198		861,654
Financial instruments sold, not yet purchased, at fair value	2,162,938		512,553
Total fair value of financial instrument liabilities	7,966,797		512,553
Equities [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	2,156,466	[1]	378,933	[2]
Financial instruments sold, not yet purchased, at fair value	1,848,729		423,740	[2]
Listed equity options [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	288,227		92,305
Financial instruments sold, not yet purchased, at fair value	229,721		69,757
Corporate debt [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	46,606		68,765
Financial instruments sold, not yet purchased, at fair value	49,616		19,056
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	160,369	[3]	7,147
Deferred compensation investments [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Deferred compensation investments	84	[4]	'
Investment in Deephaven Funds [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument assets	1,430	[4]	199,632	[5]
Securitized HECM loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	5,841,664	[6]	'
Loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	177,674		114,872
U.S. government obligations [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	32,678		'
Financial instruments sold, not yet purchased, at fair value	29,441		'
Foreign currency forward contracts [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments sold, not yet purchased, at fair value	5,431		'
Liability to GNMA trusts, at fair value [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument liabilities	5,803,859	[6]	'
Level 1 [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	2,523,977		654,875
Total fair value of financial instrument assets	2,684,346		861,654
Financial instruments sold, not yet purchased, at fair value	2,157,507		512,553
Total fair value of financial instrument liabilities	2,157,507		512,553
Level 1 [Member] | Equities [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	2,156,466	[1]	378,933	[2]
Financial instruments sold, not yet purchased, at fair value	1,848,729	[1]	423,740	[2]
Level 1 [Member] | Listed equity options [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	288,227		92,305
Financial instruments sold, not yet purchased, at fair value	229,721		69,757
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	46,606		68,765
Financial instruments sold, not yet purchased, at fair value	49,616		19,056
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	160,369	[3]	7,147
Level 1 [Member] | Deferred compensation investments [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Deferred compensation investments	0	[4]	'
Level 1 [Member] | Investment in Deephaven Funds [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument assets	0	[4]	199,632	[5]
Level 1 [Member] | Securitized HECM loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0	[6]	'
Level 1 [Member] | Loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0		114,872
Level 1 [Member] | U.S. government obligations [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	32,678		'
Financial instruments sold, not yet purchased, at fair value	29,441		'
Level 1 [Member] | Foreign currency forward contracts [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments sold, not yet purchased, at fair value	0		'
Level 1 [Member] | Liability to GNMA trusts, at fair value [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument liabilities	0	[6]	'
Level 2 [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0		0
Total fair value of financial instrument assets	6,020,852		0
Financial instruments sold, not yet purchased, at fair value	5,431		0
Total fair value of financial instrument liabilities	5,809,290		0
Level 2 [Member] | Equities [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0	[1]	0	[2]
Financial instruments sold, not yet purchased, at fair value	0	[1]	0	[2]
Level 2 [Member] | Listed equity 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] | Corporate debt [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0		0
Financial instruments sold, not yet purchased, at fair value	0		0
Level 2 [Member] | 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	[3]	'
Level 2 [Member] | Deferred compensation investments [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Deferred compensation investments	84	[4]	'
Level 2 [Member] | Investment in Deephaven Funds [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument assets	1,430	[4]	'	[5]
Level 2 [Member] | Securitized HECM loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	5,841,664	[6]	'
Level 2 [Member] | Loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	177,674		'
Level 2 [Member] | U.S. government obligations [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] | Foreign currency forward contracts [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments sold, not yet purchased, at fair value	5,431		'
Level 2 [Member] | Liability to GNMA trusts, at fair value [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument liabilities	5,803,859	[6]	'
Level 3 [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0		0
Total fair value of financial instrument assets	0		0
Financial instruments sold, not yet purchased, at fair value	0		0
Total fair value of financial instrument liabilities	0		0
Level 3 [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 3 [Member] | Listed equity 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 3 [Member] | Corporate debt [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0		0
Financial instruments sold, not yet purchased, at fair value	0		0
Level 3 [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	[3]	'
Level 3 [Member] | Deferred compensation investments [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Deferred compensation investments	0	[4]	'
Level 3 [Member] | Investment in Deephaven Funds [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument assets	0	[4]	0	[5]
Level 3 [Member] | Securitized HECM loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0	[6]	'
Level 3 [Member] | Loan inventory [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments owned, at fair value	0		0
Level 3 [Member] | U.S. government obligations [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 3 [Member] | Foreign currency forward contracts [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Financial instruments sold, not yet purchased, at fair value	0		'
Level 3 [Member] | Liability to GNMA trusts, at fair value [Member]	'		'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Total fair value of financial instrument liabilities	$0	[6]	'
[1]	Equities of $1.22 billion have been netted by their respective CUSIP number and their long and short positions.
[2]	Equities of $5.9 million have been netted by their respective CUSIP number and their long and short positions.
[3]	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.
[4]	Deferred compensation investments and Investment in Deephaven Funds are included within Investments on the Consolidated Statements of Financial Condition
[5]
[6]	Represents HECMs that have been securitized into HECM Mortgage Backed Securities (bHMBSb) where the securitization is not accounted for as a sale of the underlying HECMs. See Securitized HECM loan inventory below for full description.

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) (Level 1 [Member], USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Level 1 [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Equities	$1,220	$5.90

Fair_Value_of_Financial_Instru4

Fair Value of Financial Instruments - Additional Information (Detail)	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
	USD ($)	GBP (£)	USD ($)
Fair Value Disclosures [Abstract]	'	'	'
Transfers of financial instruments between levels	$0	'	$0
Transfers of financial insturments between level 2 to level 1	0	'	0
Transfers of financial insturments out of level 3	0	'	0
Notional value of foreign currency forward	'	£ 80,000,000	'

Fair_Value_of_Financial_Instru5

Fair Value of Financial Instruments - Fair Value of Derivative Instruments in Consolidated Statements of Financial Condition and Effect of Changes in Fair Value on Consolidated Statements of Operations (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	$295,166		$97,283
Asset Contract	803,158		244,380
Liabilities Fair Value	239,011		75,390
Liabilities Contract	765,437		260,356
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Foreign Exchange Future [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	21		8
Asset Contract	651		500
Liabilities Fair Value	62		0
Liabilities Contract	244		95
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Equity Future Contracts [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	1,442		498
Asset Contract	3,059		1,027
Liabilities Fair Value	595		70
Liabilities Contract	1,046		234
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Equity Swap [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	230		570
Asset Contract	1		1
Liabilities Fair Value	0		0
Liabilities Contract	0		0
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Fixed Income Futures Contracts [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	4,183		3,474
Asset Contract	24,400		42,143
Liabilities Fair Value	1,724		5,043
Liabilities Contract	6,601		61,398
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments not designated as hedging instruments [Member] | Commodity Contract [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	732		428
Asset Contract	2,278		1,385
Liabilities Fair Value	1,478		410
Liabilities Contract	3,565		1,824
Receivables From/Payables to Brokers, Dealers and Clearing Organizations [Member] | Derivative instruments designated as hedging instruments [Member] | Foreign currency forward contracts [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	331	[1]	'
Asset Contract	1	[1]	'
Liabilities Fair Value	0	[1]	'
Liabilities Contract	0	[1]	'
Financial Instruments Owned, at Fair Value [Member] | Derivative instruments not designated as hedging instruments [Member] | Listed equity options [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	288,227		92,305
Asset Contract	772,768		199,324
Liabilities Fair Value	229,721		69,757
Liabilities Contract	753,979		196,804
Financial Instruments Owned, at Fair Value [Member] | Derivative instruments designated as hedging instruments [Member] | Foreign currency forward contracts [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	0	[1]	'
Asset Contract	0	[1]	'
Liabilities Fair Value	5,431	[1]	'
Liabilities Contract	2	[1]	'
Accrued Expenses and Other Liabilties [Member] | Derivative instruments not designated as hedging instruments [Member] | Foreign currency forward contracts [Member]	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Asset Fair Value	'		0
Asset Contract	'		0
Liabilities Fair Value	'		110
Liabilities Contract	'		$1
[1]	Designated as hedging instrument

Fair_Value_of_Financial_Instru6

Fair Value of Financial Instruments - Fair Value of Derivative Instruments Gain Loss Recognized (Detail) (Trading Revenues, Net [Member], USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012
Derivative instruments not designated as hedging instruments [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Derivative instruments not designated as hedging instruments, Gain (Loss)	$80,996	$66,133	$235,709		$217,787
Derivative instruments not designated as hedging instruments [Member] | Foreign currency forward contracts [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Derivative instruments not designated as hedging instruments, Gain (Loss)	3,825	4,493	9,243		16,446
Derivative instruments not designated as hedging instruments [Member] | Equity Future Contracts [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Derivative instruments not designated as hedging instruments, Gain (Loss)	17,461	10,724	44,125		62,241
Derivative instruments not designated as hedging instruments [Member] | Equity Swap [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Derivative instruments not designated as hedging instruments, Gain (Loss)	1,884	1,985	10,755		5,007
Derivative instruments not designated as hedging instruments [Member] | Stock Option [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Derivative instruments not designated as hedging instruments, Gain (Loss)	17,044	24,582	63,322	[1]	41,455	[1]
Derivative instruments not designated as hedging instruments [Member] | Fixed Income Futures Contracts [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Derivative instruments not designated as hedging instruments, Gain (Loss)	22,008	15,557	64,211		66,573
Derivative instruments not designated as hedging instruments [Member] | Commodity Contract [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Derivative instruments not designated as hedging instruments, Gain (Loss)	18,774	8,792	44,053		26,065
Derivative instruments designated as hedging instruments [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Accumulated other comprehensive income (loss)	'	'	-4,948		0
Derivative instruments designated as hedging instruments [Member] | Foreign currency forward contracts [Member]	'	'	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'		'
Accumulated other comprehensive income (loss)	($4,948)	$0	'		'
[1]	Realized gains and losses on listed 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 revenue, net on the Companybs Consolidated Statements of Operations

Fair_Value_of_Financial_Instru7

Fair Value of Financial Instruments - Fair Value of Gross amounts offset (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Offsetting Liabilities [Line Items]	'	'
Gross Amounts of Recognized, Asset	$1,659,148	$144,566
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	1,659,148	144,566
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	1,329,508	50,717
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0	0
Net Amount, Assets	329,640	93,849
Gross Amounts of Recognized Derivatives, Liability	1,373,633	69,757
Gross Amounts of Recognized Derivatives, Liability	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	1,373,633	69,757
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	1,128,288	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	5,064	1,393
Net Amount, Liability	240,281	68,364
Financial instruments sold under agreements to repurchase	595,029	0
Listed equity options [Member]	'	'
Offsetting Liabilities [Line Items]	'	'
Gross Amounts of Recognized, Asset	288,227	92,305
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	288,227	92,305
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0	0
Net Amount, Assets	288,227	92,305
Gross Amounts of Recognized Derivatives, Liability	229,722	69,757
Gross Amounts of Recognized Derivatives, Liability	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	229,722	69,757
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	0	0
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	5,064	1,393
Net Amount, Liability	224,658	68,364
Securities borrowed [Member]	'	'
Offsetting Liabilities [Line Items]	'	'
Gross Amounts of Recognized, Asset	1,370,921	52,261
Gross Amounts Offset in the Statements of Financial Condition, Assets	0	0
Net Amounts of Assets Presented in the Statements of Financial Condition, Assets	1,370,921	52,261
Gross Amounts Not Offset in the Statement of Financial Position, Financial Instruments, Assets	1,329,508	50,717
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Assets	0	'
Net Amount, Assets	41,413	1,544
Securities loaned [Member]	'	'
Offsetting Liabilities [Line Items]	'	'
Gross Amounts of Recognized Derivatives, Liability	543,451	'
Gross Amounts of Recognized Derivatives, Liability	0	'
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	543,451	'
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	533,266	'
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0	'
Net Amount, Liability	10,185	'
Repurchase Agreements [Member]	'	'
Offsetting Liabilities [Line Items]	'	'
Gross Amounts of Recognized Derivatives, Liability	595,029	'
Gross Amounts of Recognized Derivatives, Liability	0	'
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	595,029	'
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	595,022	'
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0	'
Net Amount, Liability	7	'
Foreign currency forward contracts [Member]	'	'
Offsetting Liabilities [Line Items]	'	'
Gross Amounts of Recognized Derivatives, Liability	5,431	'
Gross Amounts of Recognized Derivatives, Liability	0	'
Net Amounts of Assets Presented in the Statements of Financial Condition, Liability	5,431	'
Gross Amounts Not Offset in the Statement of Financial Condition, Financial Instruments, Liability	0	'
Gross Amounts Not Offset in the Statement of Financial Position, Cash Collateral Received, Liability	0	'
Net Amount, Liability	$5,431	'

Collateralized_Transactions_Fi

Collateralized Transactions - Financial Instruments at Fair Value Received as Collateral that were Permitted to be Delivered or Repledged (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Securities Financing Transaction [Line Items]	'	'
Collateral permitted to be delivered or repledged	$6,803,143	$52,261
Fair value of financial instruments received as collateral [Member]	'	'
Securities Financing Transaction [Line Items]	'	'
Collateral permitted to be delivered or repledged	1,317,369	50,717
Collateral that was delivered or repledged	1,210,408	50,717
Collateral permitted to be further repledged by the receiving counterparty	$149,891	$0

Collateralized_Transaction_Add

Collateralized Transaction - Additional Information (Detail)	Sep. 30, 2013
Collateralized Agreements [Abstract]	'
Repurchase agreements and other secured financings, maturity (years)	'within one year

Collateralized_Transactions_Fi1

Collateralized Transactions - Financial Instruments Owned and Pledged to Counterparties that Do Not Have Right to Sell or Repledge (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Collateralized Agreements [Line Items]	'		'
Collateral permitted to be delivered or repledged	$6,803,143		$52,261
Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	732,481	[1]	0	[1]
Assets Within Discontinued Operations [Member]	'		'
Collateralized Agreements [Line Items]	'		'
Financial instruments owned, at fair value, pledged to counterparties that do not have the right to deliver or repledge	$5,840,000		'
[1]	Financial instruments owned, at fair value, pledged to counterparties that had the right to deliver or repledge includes $5.84 billion recorded within Assets within discontinued operations as of September 30, 2013.

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 $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Receivable/Payable:	'	'
Clearing organizations and other, Receivable	$1,096,308	$140,089
Securities failed to deliver, Receivable	233,805	2,880
Total Receivable	1,330,113	142,969
Clearing organizations and other, Payable	564,592	15,879
Securities failed to receive, Payable	101,586	8,306
Total Payable	$666,178	$24,185

Investments_Details

Investments (Details) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'		'
Investments accounted for under the equity method	$82,476		$0
Common stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost	41,899		48,806
Total Strategic investments	124,375		48,806
Deferred compensation investments	84		0
Knight preferred shares	0	[1]	199,632	[1]
Investment in Deephaven Funds	1,430		0
Total Investments	$125,889		$248,438
Percentage of equity in common stock of private companies cover under strategic investment	20.00%		20.00%
[1]	September 30, 2013B December 31, 2012Strategic investments: Investments accounted for under the equity method$82,476B $bCommon stock or equivalent of companies representing less than 20% equity ownership held at adjusted cost41,899B 48,806Total Strategic investments124,375B 48,806Deferred compensation investments84B bKnight preferred sharesbB 199,632Investment in Deephaven Funds1,430B bTotal Investments$125,889B $248,438

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets - Additional Information (Detail) (USD $)	9 Months Ended	12 Months Ended	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2013	Jul. 02, 2013
			Minimum [Member]	Maximum [Member]	Knight [Member]
Finite-Lived Intangible Assets [Line Items]	'	'	'	'	'
Amount	'	'	'	'	$156,800
Goodwill	$18,398	$4,645	'	'	$13,753
Amortization period, in years	'	'	'4 years	'12 years	'
Weighted average useful life	'7 years	'10 years	'	'	'

Goodwill_and_Intangible_Assets3

Goodwill and Intangible Assets - Summary of Goodwill (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Goodwill [Line Items]	'	'
Goodwill	$18,398	$4,645
Market Making [Member]	'	'
Goodwill [Line Items]	'	'
Goodwill	15,159	4,645
Global Execution Services [Member]	'	'
Goodwill [Line Items]	'	'
Goodwill	$3,239	$0

Goodwill_and_Intangible_Assets4

Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Thousands, unless otherwise specified
Goodwill [Line Items]	'		'
Gross carrying amount	$214,208		$57,408
Accumulated amortization	-22,163		-11,285
Net carrying amount	192,045		46,123
Market Making [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	99,856		46,123
Electronic Execution Services [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	77,939		0
Trading rights [Member]	'		'
Goodwill [Line Items]	'		'
Gross carrying amount	59,628	[1]	51,828	[1]
Accumulated amortization	-12,563	[1]	-9,193	[1]
Net carrying amount	47,065	[1]	42,635	[1]
Trading rights [Member] | Market Making [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	47,065		42,635
Technology [Member]	'		'
Goodwill [Line Items]	'		'
Gross carrying amount	115,580	[2]	5,580	[2]
Accumulated amortization	-8,639	[2]	-2,092	[2]
Net carrying amount	106,941	[2]	3,488	[2]
Technology [Member] | Market Making [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	52,791		3,488
Technology [Member] | Electronic Execution Services [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	39,900		0
Technology [Member] | Corporate and Other [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	14,250		'
Customer and broker relationships [Member]	'		'
Goodwill [Line Items]	'		'
Gross carrying amount	35,000	[3]	0	[3]
Accumulated amortization	-861	[3]	0	[3]
Net carrying amount	34,139	[3]	0	[3]
Customer and broker relationships [Member] | Electronic Execution Services [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	34,139		0
Trade names [Member]	'		'
Goodwill [Line Items]	'		'
Gross carrying amount	4,000	[4]	0	[4]
Accumulated amortization	-100	[4]	0	[4]
Net carrying amount	3,900	[4]	0	[4]
Trade names [Member] | Electronic Execution Services [Member]	'		'
Goodwill [Line Items]	'		'
Net carrying amount	$3,900		$0
[1]	Trading rights provide the Company with the rights to trade on certain exchanges. The weighted average remaining life is approximately 9 and 10 years as of SeptemberB 30, 2013 and DecemberB 31, 2012, respectively.
[2]	The weighted average remaining life is approximately five and three years as of SeptemberB 30, 2013 and DecemberB 31, 2012, respectively.
[3]	Customer relationships relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of SeptemberB 30, 2013. Lives may be reduced depending upon actual retention rates.
[4]	Trade names relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of SeptemberB 30, 2013.

Goodwill_and_Intangible_Assets5

Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Goodwill [Line Items]	'	'
Weighted average useful life	'7 years	'10 years
Trading rights [Member]	'	'
Goodwill [Line Items]	'	'
Amortization period, in years	'9 years	'10 years
Technology [Member]	'	'
Goodwill [Line Items]	'	'
Amortization period, in years	'5 years	'3 years
Customer and broker relationships [Member]	'	'
Goodwill [Line Items]	'	'
Amortization period, in years	'10 years	'
Trade names [Member]	'	'
Goodwill [Line Items]	'	'
Amortization period, in years	'10 years	'

Goodwill_and_Intangible_Assets6

Goodwill and Intangible Assets - Summary of Amortization Expense Relating to Intangible Assets (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Goodwill and Intangible Assets Disclosure [Abstract]	'	'	'	'
Amortization expense	$8,119	$1,380	$10,878	$4,139

Goodwill_and_Intangible_Assets7

Goodwill and Intangible Assets - Summary of Estimated Amortization Expense for Future Years (Detail) (USD $)	Sep. 30, 2013
In Thousands, unless otherwise specified
Goodwill and Intangible Assets Disclosure [Abstract]	'
For the three months ending December 31, 2013	$8,119
For the year ended December 31, 2014	32,477
For the year ended December 31, 2015	31,780
For the year ended December 31, 2016	31,082
For the year ended December 31, 2017	$31,082

LongTerm_Debt_Summary_of_LongT

Long-Term Debt - Summary of Long-Term Debt (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Debt Disclosure [Abstract]	'	'
Notes, Carrying Value	$0	$15,000
Notes, Fair Value	0	15,000
Senior Secured Notes, Carrying Value	305,000	0
Senior Secured Notes, Fair Value	299,663	0
Credit Agreement, Carrying Value	535,000	0
Credit Agreement, Fair Value	535,000	0
Convertible Notes, Carrying Amount	117,259	0
Convertible Notes, Fair Value	117,845	0
Total, Carrying Amount	957,259	15,000
Total, Fair Value	952,508	15,000
Less: Current portion recorded in Accrued expenses and other liabilities, Carrying Amount	235,000	0
Less: Current portion recorded in Accrued expenses and other liabilities, Fair Value	235,000	0
Total Long-term debt, Carrying Amount	722,259	15,000
Total Long-term debt, Fair Value	$717,508	$15,000

LongTerm_Debt_Additional_Infor

Long-Term Debt - Additional Information (Detail) (USD $)

0 Months Ended

1 Months Ended

9 Months Ended

1 Months Ended

1 Months Ended

9 Months Ended

9 Months Ended

1 Months Ended

1 Months Ended

0 Months Ended

Aug. 05, 2013

Jul. 31, 2013

Mar. 31, 2010

Sep. 30, 2013

Sep. 30, 2012

Jun. 30, 2013

Dec. 31, 2012

Jul. 31, 2013

Oct. 31, 2011

Jun. 30, 2013

Sep. 30, 2013

Jun. 05, 2013

Sep. 30, 2013

Jul. 02, 2013

Jul. 02, 2013

Jul. 02, 2013

Jul. 31, 2013

Sep. 30, 2013

Jul. 02, 2013

Jul. 31, 2013

Jul. 02, 2013

Jul. 02, 2013

Jul. 02, 2013

Jun. 30, 2013

Jun. 30, 2013

Jul. 02, 2013

Oct. 23, 2013

Oct. 23, 2013

Sep. 30, 2013

Notes Payable to Banks [Member]

Notes Payable to Banks [Member]

Secured Debt [Member]

Secured Debt [Member]

Secured Debt [Member]

Swingline Facilty [Member]

Swingline Facilty [Member]

Revolving Credit Facility [Member]

Revolving Credit Facility [Member]

Secured Debt [Member]

Secured Debt [Member]

Secured Debt [Member]

Secured Debt [Member]

Secured Debt [Member]

LIBOR Floor [Member]

Prime Rate [Member]

Borrowing Base A [Member]

Borrowing Base B Loan [Member]

LIBOR [Member]

Subsequent Event [Member]

Subsequent Event [Member]

Other Assets [Member]

OCTEG-KCA [Member]

OCTEG-KCA [Member]

OCTEG-KCA [Member]

OCTEG-KCA [Member]

OCTEG-KCA [Member]

Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

OCTEG-KCA [Member]

Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

OCTEG-KCA [Member]

OCTEG-KCA [Member]

Secured Debt [Member]

Secured Debt [Member]

Revolving Credit Facility [Member]

Revolving Credit Facility [Member]

Secured Debt [Member]

Secured Debt [Member]

Revolving Credit Facility [Member]

Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

OCTEG-KCA [Member]

OCTEG-KCA [Member]

Jefferies Finance LLC and Goldman Sachs Bank USA [Member]

OCTEG-KCA [Member]

Line of Credit Facility [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt amount

'

'

'

'

'

'

'

'

$15,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest rate

'

'

'

'

'

'

'

'

5.95%

'

'

8.25%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of days to issue securities

'

'

'

'

'

'

'

'

'

'

'365 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Early repayment fee

'

'

'

'

'

'

'

3,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Term credit agreement

'

'

'

535,000,000

'

'

0

'

'

'

'

305,000,000

'

'

'

'

'

'

535,000,000

'

450,000,000

'

'

'

'

'

'

'

'

Escrow agent fees and expenses

'

'

'

'

'

'

'

'

'

3,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional interest percent obligation from guarantor if certain obligations are not met

'

'

'

'

'

'

'

'

'

'

1.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Revolving credit facility amount

'

'

'

'

'

'

'

'

'

'

'

'

'

50,000,000

50,000,000

750,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Amount outstanding

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

increase in line of credit facility, maximum borrowing capacity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

300,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest rate of credit agreement

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.25%

2.25%

'

'

1.00%

'

'

'

Percentage points added to interest rate base

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4.50%

'

'

'

0.50%

1.75%

2.25%

3.50%

'

'

'

Interest rate at end of period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5.75%

'

'

'

'

'

'

'

'

'

'

'

Principal payments in 2014

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

235,000,000

'

'

'

'

'

'

'

'

'

'

Periodic principal payment

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

7,500,000

'

'

'

'

'

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.35%

'

'

'

'

'

'

'

'

'

Prepayment premium percentage

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.00%

'

'

'

'

'

'

'

'

'

'

'

Loan guarantee percent

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

Debt issuance costs incurred

'

'

'

'

'

'

'

'

'

'

34,600,000

'

1,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Effective interest rate

'

'

'

'

'

'

'

'

'

'

9.00%

'

'

'

'

'

'

7.60%

'

'

'

'

'

'

'

'

'

'

'

Early principal payments

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

200,000,000

200,000,000

'

Issue of cash convertible senior subordinated notes

'

'

375,000,000

'

'

375,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Cash convertible senior subordinated notes due date

'

'

15-Mar-15

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Notes bear interest rate per year

'

'

'

3.50%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Date of commencing of notes

'

'

'

15-Sep-10

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Date of mature of notes

'

'

'

15-Mar-15

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Deposit cash made to credit facilty to repurchase convertible notes

'

375,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Repayments of convertible debt

257,700,000

'

'

257,741,000

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Interest payable

3,600,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Cash collateral for borrowed securities

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$117,300,000

LongTerm_Debt_Recorded_Expense

Long-Term Debt - Recorded Expenses with Respect to Long-Term Debt (Detail) (USD $)	3 Months Ended				9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Debt Disclosure [Abstract]	'		'		'		'
Interest expense	$19,173		$688		$21,366		$1,134
Amortization of debt issuance cost	2,007	[1]	103	[1]	2,479	[1]	152	[1]
Commitment fee	267	[1]	390	[1]	9,204	[1]	390	[1]
Total	$21,447		$1,181		$33,049		$1,676
[1]	Included in Other expense.

StockBased_Compensation_Additi

Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended			9 Months Ended		3 Months Ended	9 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Jul. 02, 2013	Sep. 30, 2013	Sep. 30, 2013
				Stock Appreciation Rights (SARs) [Member]	Class B Units [Member]	Class E Units [Member]	Class E Units [Member]	KCG Plan [Member]	Restricted Stock [Member]	Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'	'	'	'	'	'	'
Number of shares authorized for grant	'	'	'	'	'	'	'	20,700,000	'	'
Percent of shares vesting over vesting period	'	'	'	'	'	'	3.33%	'	'	'
Vesting period	'	'	'	'	'3 years	'	'3 years	'	'3 years	'3 years
Unamortized compensation costs	'	$1,300,000	'	$7,200,000	$9,400,000	'	'	'	'	'
Award expiration period	'	'	'	'2 years 2 months 12 days	'	'	'5 years	'	'	'
Unamortized compensation related to restricted awards outstanding	'	'	'	'	'	'	'	'	79,600,000	'
Cost of unvested awards expected to be recognized over a weighted average life, years	'	'	'	'	'	'	'	'	'2 years 6 months	'
Number of Stock Options, Exercised (in shares)	0	'	'	'	'	'	'	'	'	'
Employee compensation deferred tax asset	'	3,000,000	3,500,000	'	'	'	'	'	'	'
Accelerated amortization	'	'	'	'	'	$0	$3,500,000	'	'	'

StockBased_Compensation_Compen

Stock-Based Compensation - Compensation Expense Relating to Restricted Awards (Detail) (Restricted Stock [Member], USD $)	3 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013
Restricted Stock [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Stock award compensation expense	$12,234
Income tax benefit	$4,649

StockBased_Compensation_Summar

Stock-Based Compensation - Summary of Restricted Awards Activity (Detail) (Restricted Stock Units [Member], USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2013	Jun. 30, 2013
Restricted Stock Units [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]	'	'	'
Number of Shares, Beginning Balance	'	8,963	0
Number of Shares, Converted	3,251	'	'
Number of Shares, Granted	6,004	'	'
Number of Shares, Vested	-45	'	'
Number of Shares, Forfeited	-246	'	'
Number of Shares, Ending Balance	'	8,963	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]	'	'	'
Weighted-Average Grant date Fair Value, Beginning Balance (in dollars per share)	'	$10.78	$0
Weighted Average Grant Date Fair Value, Number of Shares, Converted (in dollars per share)	$11.22	'	'
Weighted-Average Grant date Fair Value, Granted (in dollars per share)	$10.58	'	'
Weighted-Average Grant date Fair Value, Vested (in dollars per share)	$11.07	'	'
Weighted-Average Grant date Fair Value, Forfeited (in dollars per share)	$11.45	'	'
Weighted-Average Grant date Fair Value, Ending Balance (in dollars per share)	'	$10.78	$0

StockBased_Compensation_Compen1

Stock-Based Compensation - Compensation Expense Relating to Stock Options (Detail) (Stock Options [Member], USD $)	3 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013
Stock Options [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'
Stock option and SAR compensation expense	$915
Income tax benefit	$348

StockBased_Compensation_Summar1

Stock-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	3 Months Ended	9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2013		Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]	'	'		'
Number of Stock Options, Beginning Balance	'	'		0
Number of Stock Options, Converstion Outstanding to options	782	'		'
Number of stock options, Granted at market value	4,324	'		'
Number of Stock Options, Exercised	0	'		'
Number of Stock Options, Forfeited or expired	-79	'		'
Number of Stock Options, Ending Balance	'	5,027		0
Number of Stock Options, Exercisable	'	703		'
Available for future grants	'	6,327	[1]	'
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]	'	'		'
Weighted-Average Exercise Price, Beginning Balance	'	$0		'
Weighted-Average Exercise Price, Granted at Market Value	$41.25	'		'
Weighted-Average Exercise Price, Granted at market value	$15.19	'		'
Weighted-Average Exercise Price, Exercised	$0	'		'
Weighted-Average Exercise Price, Forfeited or expired	$45.81	'		'
Weighted-Average Exercise Price, Ending Balance	'	$18.76		'
Weighted-Average Exercise Price, Exercisable	'	$40.74		'
Aggregate Intrinsic Value of Options, Outstanding	'	$387		'
Aggregate Intrinsic Value of Options, Exercised	'	$381		'
Weighted-Average Remaining Life, Outstanding	'	'4 years 5 months 27 days		'
Weighted-Average Remaining Life, Exercisable	'	'2 years 7 months 10 days		'
Stock Appreciation Rights (SARs) [Member]	'	'		'
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]	'	'		'
Number of stock options, Granted at market value	1,700	'		'
[1]	Represents both options and awards available for grant.

StockBased_Compensation_Incent

Stock-Based Compensation - Incentive Units (Details) (Incentive Units [Member], Common Class A [Member])	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013
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, shares	24
Issued, shares	1
Number of Shares, Vested	-53
Exercised, shares	-22
Canceled, shares	-1
Ending balance, shares	55
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]	'
Number of Shares, Beginning Balance	45
Number of Shares, Granted	12
Number of Shares, Forfeited	-4
Number of Shares, Ending Balance	0

StockBased_Compensation_Class_

Stock-Based Compensation - Class B Units (Details) (Class B Units [Member], USD $)	9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013
Class B Units [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]	'
Number of Shares, Beginning Balance	417
Number of Shares, Granted	58
Number of Shares, Vested	-438
Number of Shares, Forfeited	-37
Number of Shares, Ending Balance	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]	'
Weighted-Average Grant date Fair Value, Beginning Balance (in dollars per share)	$89.99
Weighted-Average Grant date Fair Value, Granted (in dollars per share)	$73.54
Weighted-Average Grant date Fair Value, Vested (in dollars per share)	$87.86
Weighted-Average Grant date Fair Value, Forfeited (in dollars per share)	$89.28
Weighted-Average Grant date Fair Value, Ending Balance (in dollars per share)	$0

StockBased_Compensation_Class_1

Stock-Based Compensation - Class E Units (Details) (Class E Units [Member], USD $)	9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013
Class E Units [Member]	'
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]	'
Number of Shares, Beginning Balance	476
Number of Shares, Granted	0
Number of Shares, Vested	-442
Number of Shares, Forfeited	-34
Number of Shares, Ending Balance	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]	'
Weighted-Average Grant date Fair Value, Beginning Balance (in dollars per share)	$36.78
Weighted-Average Grant date Fair Value, Granted (in dollars per share)	$0
Weighted-Average Grant date Fair Value, Vested (in dollars per share)	$37.24
Weighted-Average Grant date Fair Value, Forfeited (in dollars per share)	$30.89
Weighted-Average Grant date Fair Value, Ending Balance (in dollars per share)	$0

StockBased_Compensation_Compen2

Stock-Based Compensation - Compensation Expense (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Compensation expense	($875)	$5,797	$21,288	$9,912
Class B and E Units [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Compensation expense	0	5,429	19,860	9,124
Incentive Units [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Compensation expense	($875)	$368	$1,428	$788

Income_Taxes_Effective_Income_

Income Taxes - Effective Income Tax Reconciliation (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Disclosure [Abstract]	'	'	'	'
U.S. federal statutory income tax (benefit) expense	$41,949	$5,114	$12,543	$12,224
Income not subject to U.S. corporate income tax	-44,790	-3,515	-14,071	-9,801
U.S. state and local income taxes, net of U.S. federal income tax effect	-391	533	82	825
Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	-103,499	0	-103,499	0
Nondeductible charges (1)	-1,114	74	-944	166
Foreign taxes	0	2,581	3,559	6,958
Other, net	78	18	-148	-4
Income tax (benefit) expense	($107,767)	$4,805	($102,478)	$10,368

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2013	Jul. 02, 2013
					State and Local Jurisdiction [Member]	Foreign Tax Authority [Member]	Knight [Member]
Income Tax Contingency [Line Items]	'	'	'	'	'	'	'
Deferred tax asset	'	'	'	'	'	'	$65,900,000
Deferred tax benefit resulting from the Company becoming subject to U.S. corporate income taxes	103,499,000	0	103,499,000	0	'	'	'
Operating loss carryforwards	89,200,000	'	89,200,000	'	'	68,700,000	'
Deferred tax assets, operating loss carryforwards	31,200,000	'	31,200,000	'	'	'	'
Deferred tax assets, valuation allowance	6,800,000	'	6,800,000	'	'	'	'
Deferred income tax asset	'	'	'	'	'	17,900,000	'
Foreign tax credit carryforwards	'	'	'	'	'	3,200,000	'
State and local offsetting valuation allowance	'	'	'	'	'	1,600,000	'
General business carry forward	2,500,000	'	2,500,000	'	'	'	'
AMT Carryforward	'	'	'	'	6,800,000	'	'
Unrecognized tax benefits that would impact effective tax rate if recognized	$1,400,000	'	$1,400,000	'	'	'	'

Accumulated_Other_Comprehensiv2

Accumulated Other Comprehensive Loss (Details) (USD $)	9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013
Increase (Decrease) in Stockholders' Equity [Roll Forward]	'
Beginning Balance	$114,319
Other comprehensive income	8,506
Amount reclassified from Accumulated Other Comprehensive Income	-127,972
Net current-period other comprehensive loss	-119,466
Ending Balance	-5,147
Unrealized gains on Available-for-sale Securities [Member]	'
Increase (Decrease) in Stockholders' Equity [Roll Forward]	'
Beginning Balance	114,319
Other comprehensive income	13,604
Amount reclassified from Accumulated Other Comprehensive Income	-127,972
Net current-period other comprehensive loss	-114,368
Ending Balance	-49
Foreign Currency translation adjustments [Member]	'
Increase (Decrease) in Stockholders' Equity [Roll Forward]	'
Beginning Balance	0
Other comprehensive income	-5,098
Amount reclassified from Accumulated Other Comprehensive Income	0
Net current-period other comprehensive loss	-5,098
Ending Balance	($5,098)

Earnings_Per_Share_Additional_

Earnings Per Share - Additional Information (Detail)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013
Earnings Per Share [Abstract]	'	'
Options excluded	28.8	28.8

Earnings_Per_Share_Reconciliat

Earnings Per Share - Reconciliation of Numerators and Denominators of Basic and Diluted (Loss) Earnings Per Computations (Detail) (USD $)	3 Months Ended		9 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]	'	'	'	'
Income from continuing operations and shares used in basic calculations	$227,621	$9,807	$138,316	$24,556
Income from continuing operations and shares used in diluted calculations	$227,621	$9,807	$138,316	$24,556
Shares used in basic calculations (in shares)	114,113	46,411	68,632	49,619
Effect of dilutive stock based awards (in shares)	660	0	223	0
(Loss) Income and shares used in diluted calculations (in shares)	114,773	46,411	68,855	49,619
Basic earnings per share from continuing operations (in dollars per share)	$1.99	$0.21	$2.02	$0.49
Diluted earnings per share from continuing operations (in dollars per share)	$1.98	$0.21	$2.01	$0.49

Significant_Clients_Additional

Significant Clients - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
	Client	Client	Client	Client
Risks and Uncertainties [Abstract]	'	'	'	'
Number of significant clients	0	0	0	0
Percentage of dollar value traded	10.00%	10.00%	'	10.00%

Recovered_Sheet1

Commitments and Contingent liabilities - Additional Information (Detail) (USD $)

0 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

0 Months Ended

0 Months Ended

0 Months Ended

Aug. 06, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Jun. 30, 2012

Apr. 02, 2013

Mar. 22, 2013

Apr. 02, 2013

Sep. 30, 2013

Apr. 02, 2013

Sep. 30, 2013

lawsuit

Operating lease expense [Member]

Operating lease expense [Member]

Operating lease expense [Member]

Operating lease expense [Member]

Facebook IPO [Member]

Facebook IPO [Member]

Facebook IPO [Member]

Customer [Member]

Customer [Member]

Parent Company [Member]

Revolving Credit Facility [Member]

director

Facebook IPO [Member]

Facebook IPO [Member]

Facebook IPO [Member]

Component of Operating Other Cost and Expense [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of putative class action lawsuits

'

'

'

2

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of derivative lawsuits

'

'

'

1

'

'

'

'

'

'

'

'

'

'

'

'

'

Raise In equity financing

$400,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of directors accused of breaking fiduciary duties

'

'

'

7

'

'

'

'

'

'

'

'

'

'

'

'

'

Pre-tax loss settlements realized

'

'

'

'

'

457,600,000

'

'

'

'

'

'

'

'

'

'

'

Restructuring charge

'

'

'

'

'

'

'

'

'

'

35,400,000

'

'

'

'

'

'

Claims limitation related to IPO

'

'

'

'

'

'

'

'

'

'

'

'

62,000,000

'

'

'

'

Claims submitted

'

'

'

'

'

'

'

'

'

'

'

13,000,000

'

2,600,000

'

10,400,000

'

Percentage claimed less than submitted

'

'

'

'

'

'

'

'

'

'

'

'

'

'

17.00%

'

'

Weighted average interest rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3.85%

Rental expense

'

8,898,000

3,240,000

15,454,000

8,865,000

'

5,600,000

2,300,000

10,400,000

6,400,000

'

'

'

'

'

'

'

Letters of credit held in escrow

'

1,000,000

'

1,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional borrowing capacity due to issuance of HECMs

'

'

'

$726,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt term

'

'

'

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

'

Recovered_Sheet2

Commitments and Contingent liabilities - Schedule of Capital Lease and Contract Obligations (Details) (USD $)	Sep. 30, 2013
In Thousands, unless otherwise specified
Commitments and Contingencies Disclosure [Abstract]	'
Through December 31, 2013	$2,885
2014	8,222
2015	2,072
Total	$13,179

Commitments_and_Contingent_Lia2

Commitments and Contingent Liabilities - Schedule of Interest Expense (Details) (USD $)	3 Months Ended		9 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Commitments and Contingencies Disclosure [Abstract]	'	'	'	'
Interest expense - Capital leases	$151	$285	$602	$1,135

Recovered_Sheet3

Commitments and Contingent liabilities - Schedule of Operating Lease and Contract Obligations (Detail) (USD $)	Sep. 30, 2013
In Thousands, unless otherwise specified
Gross Lease Obligations [Member]	'
Sale Leaseback Transaction [Line Items]	'
Three months ending December 31, 2013	$8,014
Year ending December 31, 2014	30,170
Year ending December 31, 2015	28,346
Year ending December 31, 2016	27,877
Year ending December 31, 2017	27,262
Thereafter through December 31, 2027	108,318
Total Amount Of Lease & Contract Obligations	229,987
Sublease Income [Member]	'
Sale Leaseback Transaction [Line Items]	'
Three months ending December 31, 2013	525
Year ending December 31, 2014	1,868
Year ending December 31, 2015	1,494
Year ending December 31, 2016	1,464
Year ending December 31, 2017	1,541
Thereafter through December 31, 2027	358
Total Amount Of Lease & Contract Obligations	7,250
Net Lease Obligations [Member]	'
Sale Leaseback Transaction [Line Items]	'
Three months ending December 31, 2013	7,489
Year ending December 31, 2014	28,302
Year ending December 31, 2015	26,852
Year ending December 31, 2016	26,413
Year ending December 31, 2017	25,721
Thereafter through December 31, 2027	107,960
Total Amount Of Lease & Contract Obligations	$222,737

Financial_instruments_with_Off1

Financial instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]	'	'
Loans outstanding	$722,259,000	$15,000,000
Executive Director or Officer [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Loans outstanding	$0	'

Business_Segments_Income_Loss_

Business Segments - Income (Loss) from Continuing Operations Before Income Taxes ("Pre-Tax Earnings") and Total Assets by Segment (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	$467,819,000	$130,602,000	$698,123,000	$425,241,000
Pre-tax earnings	119,854,000	14,612,000	35,838,000	34,924,000
Total assets	7,194,995,000	1,773,305,000	7,194,995,000	1,773,305,000
Discontinued operations assets	'	6,100,000,000	'	6,100,000,000
Market Making [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	240,110,000	107,672,000	455,678,000	383,203,000
Pre-tax earnings	47,853,000	8,214,000	55,608,000	37,976,000
Total assets	3,882,763,000	1,507,881,000	3,882,763,000	1,507,881,000
Global Execution Services [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	91,366,000	9,258,000	113,701,000	27,248,000
Pre-tax earnings	-16,354,000	-1,120,000	-21,248,000	-3,839,000
Total assets	1,426,824,000	25,934,000	1,426,824,000	25,934,000
Corporate and Other [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	136,343,000	13,672,000	128,744,000	14,790,000
Pre-tax earnings	88,355,000	7,518,000	1,478,000	787,000
Total assets	$1,885,408,000	$239,490,000	$1,885,408,000	$239,490,000

Subsequent_Events_Details

Subsequent Events (Details) (Subsequent Event [Member], USD $)	Oct. 15, 2013	Oct. 23, 2013	Oct. 16, 2013	Oct. 23, 2013	Oct. 16, 2013	Oct. 15, 2013
In Millions, unless otherwise specified		Employee Severance [Member]	Settlement with SEC [Member]	Revolving Credit Facility [Member]	Other Assets [Member]	Other Assets [Member]
Subsequent Event [Line Items]	'	'	'	'	'	'
Consents from holders of senior secured notes	$304	'	'	'	'	'
Consents from holders of senrior secured notes, percentage of total notes	99.68%	'	'	'	'	'
Prepayment period	'	'	'	'	'60 days	'
Consent solicitation fees	'	'	'	'	'	2.8
Principal payment	'	'	'	200	'	'
First periodic payment amount	'	'	'	235	'	'
Periodic payment	'	'	'	7.5	'	'
Reduction in first periodic payment by prepayment amount	'	'	'	35	'	'
Cash held in collateral account	'	'	'	117.8	'	'
Civil payment	'	'	12	'	'	'
Percentage of workforce reduction	'	5.00%	'	'	'	'
Workforce reduction charge	'	$9.40	'	'	'	'