Related Parties - Additional Information (Detail)	Jun. 30, 2013 Investor
Related Party Transactions [Abstract]
Number of investors	3
Outstanding Class A common stock	10.00%

Minimum Percentage Of Common Stock Held By Major Investors

No definition available.

Name:	kcg_MinimumPercentageOfCommonStockHeldByMajorInvestors
Namespace Prefix:	kcg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant