Merger of GETCO and Knight (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Jul. 02, 2013
Merger Transaction [Line Items]
Right of Knight shareholders to elect to receive per share value in cash under the agreement (in dollars per share)		$ 3.75
Cash paid for acquisition			$ 720.0
Shares granted to former shareholders (in shares)		41,900,000
Jefferies [Member]
Merger Transaction [Line Items]
Maximum cash entitled by shareholders to receive under the agreement		720.0
GETCO [Member]
Merger Transaction [Line Items]
Number of shares to be received by GETCO members		75,900,000
Number of warrants to acquire shares of common stock	24,300,000	24,300,000
Shares outstanding (in shares)			116,800,000
Warrants one [Member]
Merger Transaction [Line Items]
Warrants receivable from holding company		8,100,000
Exercise price of warrant (in dollars per share)	$ 12.00	$ 12.00
Term of warrant		4 years
Warrants two [Member]
Merger Transaction [Line Items]
Warrants receivable from holding company		8,100,000
Exercise price of warrant (in dollars per share)	$ 13.50	$ 13.50
Term of warrant		5 years
Warrants three [Member]
Merger Transaction [Line Items]
Warrants receivable from holding company		8,100,000
Exercise price of warrant (in dollars per share)	$ 15.00	$ 15.00
Term of warrant		6 years
Knight Class A Common Stock [Member]
Merger Transaction [Line Items]
Par Value (in dollars per share)	$ 0.01	$ 0.01
KCG Class A Common Stock [Member]
Merger Transaction [Line Items]
Par Value (in dollars per share)	$ 0.01	$ 0.01
Knight [Member]
Merger Transaction [Line Items]
Deferred tax asset			65.9
Merger costs	1.2	27.6
Gain recognized	$ 128.0	$ 128.0