Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Nov. 08, 2013 Common Class A [Member]	Nov. 08, 2013 Common Class B [Member]
Entity Information [Line Items]
Entity Registrant Name	KCG HOLDINGS, INC.
Entity Central Index Key	0001569391
Document Type	10-Q
Document Period End Date	Sep. 30, 2013
Amendment Flag	false
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		122,430,198	0