Condensed Financial Statements of KCG Holdings, Inc. (parent only) - Effects of Prior Year Misstatements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Net cash provided by (used in) operating activities	$ 12,984	$ 242,524	$ 141,204
Net cash provided by investing activities	328,478	11,472	582,892
Net cash used in financing activities	(337,336)	(349,925)	(478,811)
Parent Company
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Net cash provided by (used in) operating activities	126,990	159,473	93,101
Net cash provided by investing activities	(2,972)	0	429,564
Net cash used in financing activities	$ (92,736)	(109,790)	(384,159)
Parent Company | As Reported
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Net cash provided by (used in) operating activities		(14,358)	(398,890)
Net cash provided by investing activities		410,626	1,002,055
Net cash used in financing activities		$ (346,585)	$ (464,659)