CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $) In Thousands, unless otherwise specified	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]
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