Acquisition of KCG Holdings, Inc. - Pro forma results (Details) - USD ($) $ in Thousands	4 Months Ended	5 Months Ended	9 Months Ended	12 Months Ended
	Apr. 15, 2015	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition, Pro Forma Information [Abstract]
Revenues				$ 1,027,982	$ 702,272	$ 796,213
Net income (loss)	$ 83,147		$ 114,343	18,898	158,340	197,490
Net income (loss) attributable to common stockholders				2,939	32,980	20,887
Pro Forma
Business Acquisition, Pro Forma Information [Abstract]
Revenues				1,528,588	2,153,008
Net income (loss)				(14,151)	443,101
Net income (loss) attributable to common stockholders				(5,219)	163,407
VFH
Business Acquisition, Pro Forma Information [Abstract]
Revenues				86,599		445
Net income (loss)				2,939	32,980	104,034
Net income (loss) attributable to common stockholders				$ 2,939	$ 32,980	$ 20,887
Advisory and other professional fees		$ 24,200
KCG
Business Acquisition, Pro Forma Information [Abstract]
Revenues		379,203
Income (loss) before income taxes		14,340
Advisory and other professional fees		$ 22,500