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