Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Goodwill [Line Items]
Gross carrying amount	$ 214,208		$ 57,408
Accumulated amortization	(22,163)		(11,285)
Net carrying amount	192,045		46,123
Market Making [Member]
Goodwill [Line Items]
Net carrying amount	99,856		46,123
Electronic Execution Services [Member]
Goodwill [Line Items]
Net carrying amount	77,939		0
Trading rights [Member]
Goodwill [Line Items]
Gross carrying amount	59,628	[1]	51,828	[1]
Accumulated amortization	(12,563)	[1]	(9,193)	[1]
Net carrying amount	47,065	[1]	42,635	[1]
Trading rights [Member] | Market Making [Member]
Goodwill [Line Items]
Net carrying amount	47,065		42,635
Technology [Member]
Goodwill [Line Items]
Gross carrying amount	115,580	[2]	5,580	[2]
Accumulated amortization	(8,639)	[2]	(2,092)	[2]
Net carrying amount	106,941	[2]	3,488	[2]
Technology [Member] | Market Making [Member]
Goodwill [Line Items]
Net carrying amount	52,791		3,488
Technology [Member] | Electronic Execution Services [Member]
Goodwill [Line Items]
Net carrying amount	39,900		0
Technology [Member] | Corporate and Other [Member]
Goodwill [Line Items]
Net carrying amount	14,250
Customer and broker relationships [Member]
Goodwill [Line Items]
Gross carrying amount	35,000	[3]	0	[3]
Accumulated amortization	(861)	[3]	0	[3]
Net carrying amount	34,139	[3]	0	[3]
Customer and broker relationships [Member] | Electronic Execution Services [Member]
Goodwill [Line Items]
Net carrying amount	34,139		0
Trade names [Member]
Goodwill [Line Items]
Gross carrying amount	4,000	[4]	0	[4]
Accumulated amortization	(100)	[4]	0	[4]
Net carrying amount	3,900	[4]	0	[4]
Trade names [Member] | Electronic Execution Services [Member]
Goodwill [Line Items]
Net carrying amount	$ 3,900		$ 0

[1] Trading rights provide the Company with the rights to trade on certain exchanges. The weighted average remaining life is approximately 9 and 10 years as of September 30, 2013 and December 31, 2012, respectively.

[2] The weighted average remaining life is approximately five and three years as of September 30, 2013 and December 31, 2012, respectively.

[3] Customer relationships relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of September 30, 2013. Lives may be reduced depending upon actual retention rates.

[4] Trade names relate to KCG Hotspot and KCG BondPoint. The weighted average remaining life is approximately 10 years as of September 30, 2013.