Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2016	Nov. 02, 2016
Document And Entity Information [Abstract]
Entity Registrant Name	KCG HOLDINGS, INC.
Entity Central Index Key	0001569391
Document Type	10-Q
Document Period End Date	Sep. 30, 2016
Amendment Flag	false
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Common Class A
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		86,157,599
Common Class B
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		0