Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 192,272,000		$ 221,796,000
Accumulated amortization	(56,989,000)		(30,301,000)
Net carrying amount	135,283,000		191,495,000
KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles	34,700,000
Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	115,804,000	[1]	120,346,000	[1]
Accumulated amortization	(36,562,000)		(14,849,000)
Net carrying amount	79,242,000		105,497,000
Amortization period, in years	3 years		4 years
Technology [Member] | KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles	13,100,000
Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	62,468,000	[2]	62,450,000	[2]
Accumulated amortization	(18,110,000)		(13,530,000)
Net carrying amount	44,358,000		48,920,000
Amortization period, in years	7 years		8 years
Indefinite-lived intangible assets	6,800,000		7,600,000
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	13,000,000	[3]	35,000,000	[3]
Accumulated amortization	(2,167,000)		(1,722,000)
Net carrying amount	10,833,000		33,278,000
Amortization period, in years	8 years		10 years
Customer Relationships [Member] | KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles	19,000,000
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,000,000	[4]	4,000,000	[4]
Accumulated amortization	(150,000)		(200,000)
Net carrying amount	850,000		3,800,000
Amortization period, in years	9 years		10 years
Trade Names [Member] | KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles	2,600,000
Market Making [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	94,900,000		102,235,000
Market Making [Member] | Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	50,542,000		53,315,000
Market Making [Member] | Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	44,358,000		48,920,000
Electronic Execution Services [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	29,883,000	[5]	75,760,000	[5]
Electronic Execution Services [Member] | Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	18,200,000		38,682,000
Electronic Execution Services [Member] | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	10,833,000		33,278,000
Electronic Execution Services [Member] | Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	850,000		3,800,000
Corporate and Other [Member] | Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 10,500,000		$ 13,500,000

[1] The weighted average remaining life for technology, including capitalized software, was approximately 3 and 4 years as of December 31, 2014 and December 31, 2013, respectively. Excluded from the December 31, 2014 balance is $13.1 million of technology assets related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", is included in Assets of business held for sale at December 31, 2014.

[2] Trading rights provide the Company with the rights to trade on certain exchanges. The weighted average remaining life of trading rights with definite useful lives was approximately 7 and 8 years as of December 31, 2014 and December 31, 2013, respectively. As of December 31, 2014 and December 31, 2013, $6.8 million and $7.6 million, respectively, of trading rights had indefinite useful lives.

[3] Customer relationships relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 8 and 10 years as of December 31, 2014 and December 31, 2013, respectively. Lives may be reduced depending upon actual retention rates. Excluded from the December 31, 2014 balance is $19.0 million of customer relationships related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", is included in Assets of business held for sale at December 31, 2014.

[4] Trade names relate to KCG BondPoint and KCG Hotspot (for December 31, 2013 only). The weighted average remaining life was approximately 9 and 10 years as of December 31, 2014 and December 31, 2013, respectively. Excluded from the December 31, 2014 balance is $2.6 million of trade name related to the KCG Hotspot which as noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", is included in Assets of business held for sale at December 31, 2014.

[5] Excluded from the December 31, 2014 balance is $34.7 million of intangibles related to the KCG Hotspot which is held for sale. As noted in Footnote 4 "Discontinued Operations & Assets and Liabilities Held for Sale", such amount is included in Assets of business held for sale at December 31, 2014.