CONSOLIDATED STATEMENT OF CHANGES IN EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Class A Common Stock	Common Stock Class A Common Stock	Additional Paid-In Capital	Retained Earnings	Treasury Stock	Accumulated other comprehensive income
Beginning balance (in shares) at Dec. 31, 2013			123,317
Beginning balance at Dec. 31, 2013	$ 1,509,537		$ 1,233	$ 1,306,549	$ 211,678		$ 1,401
Beginning balance (in shares) at Dec. 31, 2013						(1,079)
Beginning balance at Dec. 31, 2013						$ (11,324)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased (in shares)						(9,570)
KCG Class A Common Stock repurchased	(111,585)					$ (111,585)
Stock-based compensation and Options & Warrants exercised (in shares)			4,191
Stock-based compensation and Options & Warrants exercised	61,907		$ 42	61,865
Income tax provision-stock based compensation	884			884
Unrealized gain (loss) on available for sale securities, net of tax	316						316
Cumulative translation adjustment, net of tax	416						416
Net income	61,102				61,102
Ending balance (in shares) at Dec. 31, 2014			127,508
Ending balance at Dec. 31, 2014	1,522,577		$ 1,275	1,369,298	272,780		2,133
Ending balance (in shares) at Dec. 31, 2014						(10,649)
Ending balance at Dec. 31, 2014						$ (122,909)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased and retired (in shares)			(23,571)
KCG Class A Common Stock repurchased and retired	(330,000)		$ (236)		(329,764)
KCG Class A Common Stock repurchased (in shares)		(3,500)				(5,220)
KCG Class A Common Stock repurchased	(63,194)	$ (41,700)				$ (63,194)
Stock-based compensation and Options & Warrants exercised (in shares)			2,088
Stock-based compensation and Options & Warrants exercised	69,188		$ 21	69,167
Income tax provision-stock based compensation	2,647			2,647
Warrants repurchased	(4,441)			(4,441)
Unrealized gain (loss) on available for sale securities, net of tax	(202)						(202)
Cumulative translation adjustment, net of tax	(1,581)						(1,581)
Net income	249,104				249,104
Ending balance (in shares) at Dec. 31, 2015			106,025
Ending balance at Dec. 31, 2015	$ 1,444,098		$ 1,060	1,436,671	192,120		350
Ending balance (in shares) at Dec. 31, 2015	(15,869)					(15,869)
Ending balance at Dec. 31, 2015	$ (186,103)					$ (186,103)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased and retired (in shares)			(18,709)
KCG Class A Common Stock repurchased and retired	(278,040)		$ (187)	(22,100)	(255,753)
KCG Class A Common Stock repurchased (in shares)		(5,500)				(7,247)
KCG Class A Common Stock repurchased	(91,240)	$ (70,700)				$ (91,240)
Stock-based compensation and Options & Warrants exercised (in shares)			2,993
Stock-based compensation and Options & Warrants exercised	39,368		$ 30	39,338
Income tax provision-stock based compensation	1,412			1,412
Warrants repurchased	(15,909)			(15,909)
Unrealized gain (loss) on available for sale securities, net of tax	3,136						3,136
Cumulative translation adjustment, net of tax	(1,239)						(1,239)
Net income	255,697				255,697
Ending balance (in shares) at Dec. 31, 2016			90,309
Ending balance at Dec. 31, 2016	$ 1,357,283		$ 903	$ 1,439,412	$ 192,064		$ 2,247
Ending balance (in shares) at Dec. 31, 2016	(23,116)					(23,116)
Ending balance at Dec. 31, 2016	$ (277,343)					$ (277,343)