Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 127,487	$ 135,283
KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 34,696
Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period	2 years	3 years
Technology [Member] | KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 13,100
Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period	6 years	7 years
Indefinite-lived intangible assets	$ 6,800	$ 6,800
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period	7 years	8 years
Customer Relationships [Member] | KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 19,000
Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization period	8 years	9 years
Trade Names [Member] | KCG Hotspot [Member]
Finite-Lived Intangible Assets [Line Items]
Held for sale, intangibles		$ 2,600
Operating Segments [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 200,773	192,272
Accumulated amortization	(73,286)	(56,989)
Operating Segments [Member] | Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	124,305	115,804
Accumulated amortization	(49,754)	(36,562)
Net carrying amount	74,551	79,242
Operating Segments [Member] | Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	62,468	62,468
Accumulated amortization	(20,443)	(18,110)
Net carrying amount	42,025	44,358
Operating Segments [Member] | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	13,000	13,000
Accumulated amortization	(2,889)	(2,167)
Net carrying amount	10,111	10,833
Operating Segments [Member] | Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,000	1,000
Accumulated amortization	(200)	(150)
Net carrying amount	800	850
Operating Segments [Member] | Market Making [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	91,976	94,900
Operating Segments [Member] | Market Making [Member] | Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	49,951	50,542
Operating Segments [Member] | Market Making [Member] | Trading Rights [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	42,025	44,358
Operating Segments [Member] | Electronic Execution Services [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	26,511	29,883
Operating Segments [Member] | Electronic Execution Services [Member] | Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	15,600	18,200
Operating Segments [Member] | Electronic Execution Services [Member] | Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	10,111	10,833
Operating Segments [Member] | Electronic Execution Services [Member] | Trade Names [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	800	850
Operating Segments [Member] | Corporate and Other [Member] | Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Net carrying amount	$ 9,000	$ 10,500