CONSOLIDATED STATEMENT OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Common Class A	Redeemable preferred member's equity	Members' Equity	Unrecognized Compensation	Common Stock Common Class A	Additional Paid-In Capital	Retained Earnings	Treasury Stock	Accumulated other comprehensive income
Beginning balance (in shares) at Dec. 31, 2012						0
Beginning balance at Dec. 31, 2012	$ 654,672		$ 311,139	$ 559,292	$ (18,939)	$ 0	$ 0	$ 0		$ 114,319
Beginning balance, Treasury stock (in shares) at Dec. 31, 2012									0
Beginning balance, Treasury stock 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)
Unrealized gain on available for sale securities, net	4,550									4,550
Cumulative translation adjustment, net	(403)									(403)
Net (loss) income	(89,305)			(89,305)
Modification of redemption value	21,565		(21,565)	21,565
Ending balance (in shares) at Jun. 30, 2013						0
Ending balance at Jun. 30, 2013	613,470		289,574	495,004	0	$ 0	0	0		118,466
Ending balance, Treasury stock (in shares) at Jun. 30, 2013									0
Ending balance, Treasury stock at Jun. 30, 2013									$ 0
Beginning balance (in shares) at Dec. 31, 2012						0
Beginning balance at Dec. 31, 2012	654,672		311,139	559,292	(18,939)	$ 0	0	0		114,319
Beginning balance, Treasury stock (in shares) at Dec. 31, 2012									0
Beginning balance, Treasury stock at Dec. 31, 2012									$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	126,172
Ending balance (in shares) (As Reported) at Dec. 31, 2013						123,317,000
Ending balance (in shares) at Dec. 31, 2013						123,317,000
Ending balance (As Reported) at Dec. 31, 2013	1,507,252		0	0		$ 1,233	1,306,549	209,393		1,401
Ending balance at Dec. 31, 2013	1,509,537					$ 1,233	1,306,549	211,678		1,401
Ending balance (Cumulative Effect of Change in Accounting Principle) at Dec. 31, 2013	2,285							2,285
Ending balance, Treasury stock (in shares) (As Reported) at Dec. 31, 2013									(1,079,000)
Ending balance, Treasury stock (in shares) at Dec. 31, 2013									(1,079,000)
Ending balance, Treasury stock (As Reported) at Dec. 31, 2013									$ (11,324)
Ending balance, Treasury stock at Dec. 31, 2013									$ (11,324)
Beginning balance (in shares) at Jun. 30, 2013						0
Beginning balance at Jun. 30, 2013	613,470		289,574	495,004	$ 0	$ 0	0	0		118,466
Beginning balance, Treasury stock (in shares) at Jun. 30, 2013									0
Beginning balance, Treasury stock at Jun. 30, 2013									$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased, shares									(1,079,000)
KCG Class A Common Stock repurchased	(11,324)								$ (11,324)
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 stockholders, shares						41,889,000
Issuance of KCG Class A Common Stock to Knight stockholders	453,838					$ 419	453,419
Change in estimated distribution payable to members	1,757						1,757
Stock-based compensation, shares						5,560,000
Stock-based compensation	22,115					$ 55	22,060
Unrealized gain on available for sale securities, net	36									36
Cumulative translation adjustment, net	1,768									1,768
Net (loss) income	209,393							209,393
Ending balance (in shares) (As Reported) at Dec. 31, 2013						123,317,000
Ending balance (in shares) at Dec. 31, 2013						123,317,000
Ending balance (As Reported) at Dec. 31, 2013	1,507,252		$ 0	$ 0		$ 1,233	1,306,549	209,393		1,401
Ending balance at Dec. 31, 2013	1,509,537					$ 1,233	1,306,549	211,678		1,401
Ending balance (Cumulative Effect of Change in Accounting Principle) at Dec. 31, 2013	2,285							2,285
Ending balance, Treasury stock (in shares) (As Reported) at Dec. 31, 2013									(1,079,000)
Ending balance, Treasury stock (in shares) at Dec. 31, 2013									(1,079,000)
Ending balance, Treasury stock (As Reported) at Dec. 31, 2013									$ (11,324)
Ending balance, Treasury stock at Dec. 31, 2013									$ (11,324)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Income tax provision-stock based compensation	884						884
Unrealized gain on available for sale securities, net	$ 316									316
Stock options exercised, shares	0
Cumulative translation adjustment, net	$ 416									416
Net (loss) income	61,102							61,102
Ending balance (in shares) at Dec. 31, 2014						127,508,000
Ending balance at Dec. 31, 2014	$ 1,522,577					$ 1,275	1,369,298	272,780		2,133
Ending balance, Treasury stock (in shares) at Dec. 31, 2014	(10,649,000)								(10,649,000)
Ending balance, Treasury stock at Dec. 31, 2014	$ (122,909)								$ (122,909)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased and retired via Tender Offer, shares						(23,571,000)
KCG Class A Common Stock repurchased and retired via Tender Offer	(330,000)					$ (236)		(329,764)
KCG Class A Common Stock repurchased, shares		(3,500,000)							(5,220,000)
KCG Class A Common Stock repurchased	(63,194)	$ (41,700)							$ (63,194)
Stock-based compensation, shares						1,883,000
Stock-based compensation	67,409					$ 19	67,390
Income tax provision-stock based compensation	2,647						2,647
Unrealized gain on available for sale securities, net	(202)									(202)
Stock options exercised, shares						151,000
Stock options exercised	1,247					$ 2	1,245
Warrants exercised, shares						54,000
Warrants exercised	532						532
Warrants repurchased	(4,441)						(4,441)
Cumulative translation adjustment, net	(1,581)									(1,581)
Net (loss) income	249,104							249,104
Ending balance (in shares) at Dec. 31, 2015						106,025,000
Ending balance at Dec. 31, 2015	$ 1,444,098					$ 1,060	$ 1,436,671	$ 192,120		$ 350
Ending balance, Treasury stock (in shares) at Dec. 31, 2015	(15,869,000)								(15,869,000)
Ending balance, Treasury stock at Dec. 31, 2015	$ (186,103)								$ (186,103)