Acquisition of KCG Holdings, Inc. (Details) - USD ($) $ / shares in Units, $ in Thousands						9 Months Ended	12 Months Ended
	Aug. 10, 2017	Jul. 21, 2017	Jul. 20, 2017	Apr. 21, 2015	Apr. 15, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 16, 2017
Business Acquisition [Line Items]
Aggregate purchase price						$ 327,366	$ 735,974
Employee compensation expense							6,400
Annual incentive compensation payments							$ 35,300
Temasek Investment
Business Acquisition [Line Items]
Stock issuance fees	$ 7,800
Senior Secured Second Lien Notes
Business Acquisition [Line Items]
Face amount										$ 500,000
Interest rate (as a percent)										6.75%
Senior Secured Notes Due 2020
Business Acquisition [Line Items]
Face amount		$ 465,000
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
Number of shares of stock issued				19,012,112	16,532,272
Employee compensation expense							$ 300	$ 10,600	$ 13,200
Class A | NIH Investment Agreement
Business Acquisition [Line Items]
Number of shares of stock issued	338,124
Aggregate purchase price	$ 5,200
VFH | Class A
Business Acquisition [Line Items]
Common stock, par value							$ 0.00001
KCG
Business Acquisition [Line Items]
Common stock, par value			$ 0.01
Right to receive cash			20.00
Fair value ( in dollars per share)			$ 20.00
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
Aggregate purchase price	$ 5,200