Business Segments - Income (Loss) from Continuing Operations Before Income Taxes ("Pre-Tax Earnings") and Total Assets by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment Reporting Information [Line Items]
Revenues	$ 315,387	[1]	$ 252,094	[1]	$ 599,690	[1]	$ 553,026	[1]
Pre-tax earnings	(21,778)	[1]	7,609	[1]	(3,315)	[1]	58,599	[1]
Total assets	11,324,207		8,737,443		11,324,207		8,737,443
Market Making [Member]
Segment Reporting Information [Line Items]
Revenues	167,878		113,490		318,607		265,655
Pre-tax earnings	49,460		5,933		86,010		51,063
Total assets	3,568,889		1,779,364		3,568,889		1,779,364
Global Execution Services [Member]
Segment Reporting Information [Line Items]
Revenues	133,741		118,876		255,261		257,607
Pre-tax earnings	714		6,341		11,989		30,079
Total assets	5,738,023		4,033,626		5,738,023		4,033,626
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	13,768		19,728		25,822		29,764
Pre-tax earnings	(71,952)		(4,665)		(101,314)		(22,543)
Total assets	$ 2,017,295		$ 2,924,453		$ 2,017,295		$ 2,924,453

[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