Goodwill and Intangible Assets - Additional Information (Detail) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Finite-Lived Intangible Assets [Line Items]
Writedown of assets	$ 17,787,000	[1]	$ 0	[1]	$ 17,787,000	[1]	$ 0	[1]
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period, in years					2 years
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period, in years					17 years
Segment, Discontinued Operations [Member]
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment charges	$ 1,200,000				$ 9,500,000

[1] See Explanatory Note - All financial information is of Knight Capital Group, Inc. As of July 1, 2013, Knight Capital Group, Inc., became a wholly-owned subsidiary of KCG