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 (loss) income
Beginning balance at Dec. 31, 2015							$ 350
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased (in shares)		(1,800)
KCG Class A Common Stock repurchased		$ (20,500)
Unrealized gain on available for sale securities, net of tax	$ 68
Reclassification of realized gain from Other comprehensive income, net of tax	0
Cumulative translation adjustment, net of tax	(133)
Net income	37,165
Ending balance at Mar. 31, 2016							285
Beginning balance (in shares) at Dec. 31, 2016			90,309
Beginning balance at Dec. 31, 2016	$ 1,357,283		$ 903	$ 1,439,412	$ 192,064		2,247
Beginning balance (in shares) at Dec. 31, 2016	(23,116)					(23,116)
Beginning balance at Dec. 31, 2016	$ (277,343)					$ (277,343)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased (in shares)		(900)				(2,029)
KCG Class A Common Stock repurchased	(28,849)	$ (13,300)				$ (28,849)
Stock-based compensation and Options & Warrants exercised (in shares)			1,501
Stock-based compensation and Options & Warrants exercised	18,015		$ 15	18,000
Unrealized gain on available for sale securities, net of tax	170						170
Reclassification of realized gain from Other comprehensive income, net of tax	(2,956)						(2,956)
Cumulative translation adjustment, net of tax	437						437
Net income	3,212				3,212
Ending balance (in shares) at Mar. 31, 2017			91,810
Ending balance at Mar. 31, 2017	$ 1,347,312		$ 918	$ 1,457,412	$ 195,276		$ (102)
Ending balance (in shares) at Mar. 31, 2017	(25,145)					(25,145)
Ending balance at Mar. 31, 2017	$ (306,192)					$ (306,192)