Capital Structure - Acquisition of KCG (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Jul. 20, 2017	May 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Aggregate purchase price			$ 735,974
Class A
Class of Stock [Line Items]
Number of shares of stock issued (in shares)		10,518,750
KCG | Class A | Aranda
Class of Stock [Line Items]
Number of shares of stock issued (in shares)	8,012,821
Aggregate purchase price	$ 125,000
KCG | Class A | North Island Stockholder
Class of Stock [Line Items]
Number of shares of stock issued (in shares)	40,064,103
Aggregate purchase price	$ 618,700