Merger of Virtu Financial, Inc. and KCG Holdings, Inc. (Details) - USD ($) $ / shares in Units, $ in Thousands								3 Months Ended		9 Months Ended
	Aug. 10, 2017	Aug. 08, 2017	Jul. 21, 2017	Jul. 20, 2017	Jun. 16, 2017	Apr. 21, 2015	Apr. 15, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Aggregate purchase price				$ 613,500
Employee compensation expense								$ 6,400		$ 6,400
Annual incentive compensation payments								23,000		$ 23,000
Temasek Investment
Business Acquisition [Line Items]
Stock issuance fees	$ 7,800
NIH Investment Agreement
Business Acquisition [Line Items]
Number of shares of stock issued	338,124
Aggregate purchase price	$ 5,200
Senior Secured Credit Facility
Business Acquisition [Line Items]
Face amount												$ 540,000
Senior Secured Credit Facility Fourth Amendment
Business Acquisition [Line Items]
Release of escrow balance				$ 1,119,400
Escrow Credit Facility
Business Acquisition [Line Items]
Face amount												610,000
Senior Secured Second Lien Notes
Business Acquisition [Line Items]
Face amount					$ 500,000
Interest rate (as a percent)					6.75%
Interest period					6 months
Redemption price as a percentage of the principal amount										100.00%
Term Loan
Business Acquisition [Line Items]
Outstanding principal balance								$ 950,000		$ 950,000
Voluntary prepayment of principal		$ 100,000
Senior Secured Notes Due 2020
Business Acquisition [Line Items]
Interest rate (as a percent)			6.875%
Redemption price as a percentage of the principal amount			103.438%
Class A
Business Acquisition [Line Items]
Common stock, par value						$ 0.00001		$ 0.00001		$ 0.00001			$ 0.00001
Number of shares of stock issued						19,012,112	16,532,272
Employee compensation expense								$ 0	$ 2,900	$ 9,400	$ 8,700
Class A | Temasek Investment
Business Acquisition [Line Items]
Number of shares of stock issued	1,666,666
Aggregate purchase price	$ 26,000
VFH | Senior Secured Credit Facility
Business Acquisition [Line Items]
Face amount												$ 100,000
KCG
Business Acquisition [Line Items]
Common stock, par value				$ 0.01
Right to receive cash				$ 20.00
Cash purchase price				$ 1,400
Fair value ( in dollars per share)				$ 20.00
Purchase price revision measurement period				12 years
KCG | NIH Investment Agreement
Business Acquisition [Line Items]
Number of shares of stock issued				39,725,979
Aggregate purchase price				$ 613,500
KCG | Class A | Aranda
Business Acquisition [Line Items]
Number of shares of stock issued				6,346,155
Aggregate purchase price				$ 99,000
KCG | Class A | Temasek Investment
Business Acquisition [Line Items]
Number of shares of stock issued	1,666,666
Aggregate purchase price	$ 26,000
KCG | Class A | NIH Investment Agreement
Business Acquisition [Line Items]
Number of shares of stock issued	338,124			39,725,979
Aggregate purchase price	$ 5,200