Share-based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended		9 Months Ended		12 Months Ended
	Apr. 22, 2015	Apr. 21, 2015	Jul. 08, 2011	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation.
Expense recognized				$ 6,400		$ 6,400
Number of Class A-2 capital interests contributed by Class A-2 members to Virtu East MIP LLC			2,625,000
Non-voting common interest units outstanding				12,765,051		12,765,051		14,231,535
Number of non-voting common interest units forfeited or repurchased						1,466,484	1,162,891
Weighted Average Remaining Life, outstanding
Options Exercisable, Weighted Average Exercise Price (in dollars per share)								$ 19.00
Class A
Share-based Compensation.
Expense recognized				$ 0	$ 2,900	$ 9,400	$ 8,700
Number of non-voting common interest units forfeited or repurchased					1,100,668
Class C
Share-based Compensation.
Number of non-voting common interest units forfeited or repurchased				429,668
2015 Management Incentive Plan | Class A | Maximum
Share-based Compensation.
Vesting period	4 years
Class A-2 profits interests
Share-based Compensation.
Expense recognized				$ 100	$ 400	500	$ 1,100
Unrecognized share-based compensation expense				$ 100		$ 100
Weighted average period for compensation expense expected to be recognized						1 month 6 days
Class A-2 profits interests | Maximum
Share-based Compensation.
Vesting period				4 years	4 years	4 years	4 years
Class A-2 profits interests | East MIP | Time based vesting
Share-based Compensation.
Vesting period			4 years
Class B interests | East MIP
Share-based Compensation.
Amortization expense related to share based compensation				$ 20	$ 100	$ 60	$ 700
Non-voting common interest units (formerly Class B interests) | East MIP
Share-based Compensation.
Expense recognized				200	200	500	700
Unrecognized share-based compensation expense				200		$ 200		$ 800
Weighted average period for compensation expense expected to be recognized						3 months 18 days		1 year
Capitalized software development costs				10	30	$ 20	60
Non-qualified stock options
Share-based Compensation.
Expense recognized						4,200	$ 4,200
Non-qualified stock options | 2015 Management Incentive Plan
Share-based Compensation.
Vesting period		4 years
Expense recognized				1,400	$ 1,500
Unrecognized share-based compensation expense				$ 9,100		$ 9,100		$ 14,200
Weighted average period for compensation expense expected to be recognized						1 year 7 months 6 days		2 years 3 months 18 days
Forfeited or repurchased (in shares)						401,000	645,000
Outstanding (in shares)						8,234,000	8,994,000	8,994,000
Forfeited or expired (in shares)						(401,000)	(645,000)
Outstanding (in shares)				7,833,000	8,349,000	7,833,000	8,349,000	8,234,000	8,994,000
Weighted Average Exercise Price Per Share
Outstanding (in dollars per share)						$ 19.00	$ 19.00	$ 19.00
Outstanding (in dollars per share)				$ 19.00	$ 19.00	$ 19.00	$ 19.00	$ 19.00	$ 19.00
Weighted Average Remaining Life, outstanding
Weighted Average Remaining Life						7 years 6 months 18 days	8 years 6 months 18 days	8 years 3 months 15 days	9 years 3 months 15 days
Options Exercisable, Number of Options (in shares)				3,916,500	2,087,250	3,916,500	2,087,250	2,058,500
Options Exercisable, Weighted Average Exercise Price (in dollars per share)				$ 19.00		$ 19.00
Non-qualified stock options | 2015 Management Incentive Plan | Maximum
Share-based Compensation.
Expiration period		10 years
Restricted stock units
Share-based Compensation.
Expense recognized				$ 2,200	$ 1,600	$ 7,100	$ 8,700
Unrecognized share-based compensation expense				$ 17,200		$ 17,200		$ 28,500
Weighted average period for compensation expense expected to be recognized						1 year 9 months 18 days		2 years 7 months 6 days
Activity
Outstanding (in shares)						1,573,441	984,466	984,466
Granted (in shares)						34,825	35,095
Forfeited (in shares)						(212,729)	(115,869)
Vested (in shares)						(58,536)	(49,088)
Outstanding (in shares)				1,337,001	854,604	1,337,001	854,604	1,573,441	984,466
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Outstanding (in dollars per share)						$ 18.28
Granted (in dollars per share)							$ 17.81
Forfeited (in dollars per share)							22.51
Vested (in dollars per share)							20.11
Outstanding (in dollars per share)					$ 22.22		22.22	$ 18.28
Weighted Average Fair Value
Outstanding (in dollars per share)							$ 22.32	$ 22.32
Granted (in dollars per share)						17.95
Forfeited (in dollars per share)						18.46
Vested (in dollars per share)						19.22
Outstanding (in dollars per share)				$ 18.21		$ 18.21			$ 22.32