Merger With GETCO and formation of KCG Holdings Inc. - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended		1 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Jefferies [Member]	Jun. 30, 2013 GETCO [Member]	Jun. 30, 2013 Warrants one [Member]	Mar. 31, 2013 Warrants two [Member]	Jun. 30, 2013 Warrants two [Member]	Jun. 30, 2013 Warrants three [Member]	Jul. 31, 2013 Subsequent Event [Member]	Jun. 30, 2013 Subsequent Event [Member]	Jun. 30, 2013 Subsequent Event [Member] GETCO [Member]	Jun. 30, 2013 Knight Class A Common Stock [Member]	Jun. 30, 2013 KCG Class A Common Stock [Member]	Jun. 30, 2013 Common Class A [Member]
Merger Transaction [Line Items]
Common Stock, Par or Stated Value Per Share												$ 0.01	$ 0.01
Right of Knight shareholders to elect to receive per share value in cash under the agreement	$ 3.75
Maximum cash entitled by shareholders to receive under the agreement			$ 720.0
Number of shares to be received by GETCO members				75,900,000
Number of warrants to acquire shares of common stock				24,000,000
Warrants receivable from holding company					8,100,000	8,100,000		8,100,000
Exercise price of warrant					$ 12.00		$ 13.50	$ 15.00
Term of warrant					4 years		5 years	6 years
Class A common stock, Shares outstanding	373,271,000	202,056,000												373,300,000
Cash paid for acquistion										$ 720.0
Shares purchased										192,000,000
Conversion ratio									3
Shares outstanding											116,800,000