Merger of Virtu Financial, Inc. and KCG Holdings, Inc. - Intangible assets (Details) - USD ($) $ in Thousands		9 Months Ended
	Jul. 20, 2017	Sep. 30, 2017	Dec. 31, 2016
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		$ 152,748	$ 992
Goodwill		859,598	715,379
Deferred tax asset		224,804	193,859
Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		76,073
Trade names
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets		4,902
Amortization Years	10 years
KCG
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 156,300
Goodwill	143,012
Total	299,312
Goodwill deductible for tax purposes	0
Deferred tax asset	22,900	$ 31,500
KCG | Technology.
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	67,800
KCG | Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	77,100
KCG | Trade names
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	5,000
KCG | Exchange memberships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Intangible assets	$ 6,400
Minimum | KCG | Technology.
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	1 year
Minimum | KCG | Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	13 years
Maximum | KCG | Technology.
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years		6 years
Maximum | KCG | Customer relationships
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Amortization Years	17 years
Market Making
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Goodwill	$ 107,200	$ 766,261	657,985
Execution Services
Amounts preliminarily allocated to intangible assets, the amortization period and goodwill
Goodwill	$ 35,800	$ 93,337	$ 57,394