CONSOLIDATED STATEMENT OF CHANGES IN EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock Class A Common Stock	Additional Paid-In Capital	Retained Earnings	Treasury Stock	Accumulated other comprehensive income
Beginning balance at Dec. 31, 2014						$ 2,133
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain on available for sale securities, net of tax	$ (175)
Cumulative translation adjustment, net of tax	(1,651)
Net income	252,076
Ending balance at Sep. 30, 2015						307
Beginning balance at Jun. 30, 2015						1,214
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain on available for sale securities, net of tax	(312)
Cumulative translation adjustment, net of tax	(595)
Net income	21,937
Ending balance at Sep. 30, 2015						307
Beginning balance (in shares) at Dec. 31, 2015		106,025
Beginning balance at Dec. 31, 2015	$ 1,444,098	$ 1,060	$ 1,436,671	$ 192,120		350
Beginning balance (in shares) at Dec. 31, 2015	(15,869)				(15,869)
Beginning balance at Dec. 31, 2015	$ (186,103)				$ (186,103)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
KCG Class A Common Stock repurchased (in shares)					(6,930)
KCG Class A Common Stock repurchased	(87,167)				$ (87,167)
Warrants repurchased	(15,909)		(15,909)
Stock-based compensation and Options & Warrants exercised (in shares)		2,987
Stock-based compensation and Options & Warrants exercised	39,581	$ 30	39,551
Unrealized gain on available for sale securities, net of tax	172					172
Cumulative translation adjustment, net of tax	(632)					(632)
Net income	59,505			59,505
Ending balance (in shares) at Sep. 30, 2016		109,012
Ending balance at Sep. 30, 2016	$ 1,439,648	$ 1,090	1,460,313	251,625		(110)
Ending balance (in shares) at Sep. 30, 2016	(22,799)				(22,799)
Ending balance at Sep. 30, 2016	$ (273,270)				$ (273,270)
Beginning balance at Jun. 30, 2016						(216)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Unrealized gain on available for sale securities, net of tax	88
Cumulative translation adjustment, net of tax	18
Net income	(11,214)
Ending balance (in shares) at Sep. 30, 2016		109,012
Ending balance at Sep. 30, 2016	$ 1,439,648	$ 1,090	$ 1,460,313	$ 251,625		$ (110)
Ending balance (in shares) at Sep. 30, 2016	(22,799)				(22,799)
Ending balance at Sep. 30, 2016	$ (273,270)				$ (273,270)