Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Feb. 24, 2016	Jun. 30, 2015
Entity Information [Line Items]
Entity Registrant Name	KCG HOLDINGS, INC.
Entity Central Index Key	0001569391
Document Type	10-K
Document Period End Date	Dec. 31, 2015
Amendment Flag	false
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 783.4
Common Class A
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		90,878,650
Common Class B
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		0