Merger of Virtu Financial, Inc. and KCG Holdings, Inc. - Estimated fair values of the assets acquired and liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jul. 20, 2017	Dec. 31, 2016
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Goodwill	$ 859,598		$ 715,379
KCG
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net [Abstract]
Cash and equivalents		$ 592,993
Cash and securities segregated under federal regs		3,000
Securities borrowed		1,406,444
Securities purchased under agreements to resell		16,894
Receivables from broker dealers and clearing organizations		553,031
Financial instruments owned, at fair value		2,095,339
Property, equipment and capitalized software (net)		112,204
Intangibles		156,300
Deferred taxes		22,928
Other assets		331,820
Total Assets		5,290,953
Securities loaned		166,189
Securities sold under agreements to repurchase		841,606
Payables to broker dealers and clearing organizations		536,653
Payables to customers		17,583
Financial instruments sold, not yet purchased, at fair value		1,756,647
Accounts payable and accrued expenses and other liabilities		239,004
Debt		480,987
Total Liabilities		4,038,669
Total identified assets acquired, net of assumed liabilities		1,252,284
Goodwill		143,012
Total Purchase Price		$ 1,395,296