Merger of Virtu Financial, Inc. and KCG Holdings, Inc. - Pro forma results (Details) - USD ($)	2 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Business Acquisition, Pro Forma Information [Abstract]
Revenues			$ 271,286,000	$ 164,806,000	$ 563,461,000	$ 531,625,000
Net income (loss)			(39,990,000)	33,023,000	(14,503,000)	123,665,000	$ 158,340,000
Net income (loss) attributable to common stockholders			$ (13,518,000)	$ 7,026,000	$ (8,037,000)	$ 25,752,000
Diluted (in dollars per share)			$ (0.17)	$ 0.18	$ (0.17)	$ 0.66
Pro Forma
Business Acquisition, Pro Forma Information [Abstract]
Revenues			$ 304,020,000	$ 373,338,000	$ 1,068,164,000	$ 1,401,819,000
Net income (loss)			(82,520,000)	1,783,000	(117,169,000)	163,146,000
Net income (loss) attributable to common stockholders			$ (30,432.00)	$ 658.00	$ (43,210.00)	$ 60,165.00
Diluted (in dollars per share)			$ (0.35)	$ 0.01	$ (0.48)	$ 0.67
VFH
Business Acquisition, Pro Forma Information [Abstract]
Advisory and other professional fees	$ 24,200,000
KCG
Business Acquisition, Pro Forma Information [Abstract]
Revenues		$ 149,327,000
Income (loss) before income taxes		$ (12,578,000)
Advisory and other professional fees	$ 22,500,000