Merger of GETCO and Knight (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	0 Months Ended	6 Months Ended
	Jul. 01, 2013	Jun. 30, 2014	Jul. 01, 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
Maximum cash entitled by shareholders to receive under the agreement	$ 720,000,000
Shares granted to former shareholders (in shares)	41.9
Knight [Member]
Merger Transaction [Line Items]
Total purchase price	1,370,000,000
Goodwill adjustments		1,100,000
Deferred tax assets adjustments		3,100,000
Accrued expenses adjustments		3,500,000
Other assets adjustments		700,000
Deferred tax asset, net	$ 65,465,000		$ 65,465,000
Warrants one [Member]
Merger Transaction [Line Items]
Warrants receivable from holding company (in shares)	8.1
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 (in shares)	8.1
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 (in shares)	8.1
Exercise price of warrant (in dollars per share)	$ 15.00		$ 15.00
Term of warrant	6 years
GETCO [Member]
Merger Transaction [Line Items]
Number of shares to be received by GETCO members (in shares)	75.9
Number of warrants to acquire shares of common stock (in shares)	24.3		24.3
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