Employee and Non-Employee Director Stock and Benefit Plans - Share Awards (Details) $ / shares in Units, $ in Millions			12 Months Ended
	Jan. 15, 2019 $ / shares shares	May 27, 2016 USD ($) shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) installment item $ / shares shares
Employee and non-employee director stock and benefit plans
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.01	$ 0.01
Common Class A [Member]
Employee and non-employee director stock and benefit plans
Trading days based on which volume-weighted average price per share is calculated			10 days
Common Class A [Member] | Virtu Financial [Member]
Employee and non-employee director stock and benefit plans
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.00001
Chief Executive Officer
Employee and non-employee director stock and benefit plans
Number of Inducement Awards granted | item					2
Restricted share awards
Total Number of Shares
Outstanding at the beginning of the period (in shares)			2,907,545	3,063,264	3,171,036
Granted (in shares)			1,307,357	1,259,392	1,665,916
Vested (in shares)			(1,277,549)	(1,331,021)	(1,042,238)
Forfeited (in shares)			(257,099)	(84,090)	(731,450)
Outstanding at the end of the period (in shares)			2,680,254	2,907,545	3,063,264
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 18.10	$ 16.63	$ 16.75
Granted (in dollars per share) | $ / shares			21.18	19.81	16.16
Vested (in dollars per share) | $ / shares			18.84	16.60	16.06
Forfeited (in dollars per share) | $ / shares			19.35	18.99	16.88
Outstanding at the end of the period (in dollars per share) | $ / shares			$ 19.50	$ 18.10	$ 16.63
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			1,277,549	1,331,021	1,042,238
Unrecognized compensation costs | $			$ 25.2
Weighted average period over which unrecognized compensation costs are expected to be recognized			1 year 2 months 12 days
Grant date fair value | $			$ 23.9
Tax benefits related to the vesting | $			0.6	$ 1.5	$ 0.8
Tax shortfalls | $			0.1	0.1	0.7
Share-based compensation expense | $			28.4	19.6	24.9
Tax benefit related to share-based compensation expense | $			$ 1.2	$ 0.9	$ 8.9
ROE-Based Market Based Or Performance Based
Total Number of Shares
Outstanding at the beginning of the period (in shares)			393,344	342,370	377,475
Granted (in shares)			122,424	140,796	228,895
Vested (in shares)			(45,065)	(51,498)	(73,243)
Forfeited (in shares)			(144,715)	(38,324)	(190,757)
Outstanding at the end of the period (in shares)			325,988	393,344	342,370
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			45,065	51,498	73,243
Performance-based share awards
Total Number of Shares
Outstanding at the end of the period (in shares)			263,220
Market-based share awards
Total Number of Shares
Outstanding at the end of the period (in shares)			62,768
Time-Based Restricted Stock Units
Total Number of Shares
Outstanding at the beginning of the period (in shares)			2,514,201	2,720,894	2,793,561
Granted (in shares)			1,184,933	1,118,596	1,437,021
Vested (in shares)			(1,232,484)	(1,279,523)	(968,995)
Forfeited (in shares)			(112,384)	(45,766)	(540,693)
Outstanding at the end of the period (in shares)			2,354,266	2,514,201	2,720,894
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			1,232,484	1,279,523	968,995
Restricted Stock, Inducement Awards, Tranche One [Member] | Chief Executive Officer
Total Number of Shares
Granted (in shares)					135,353
Additional disclosures
Number of equal annual vesting installments | installment					3
Restricted Stock, Inducement Awards, Tranche Two [Member] | Chief Executive Officer
Total Number of Shares
Granted (in shares)					156,051
Restricted Stock, Inducement Awards, Tranche Two [Member] | Chief Executive Officer | Vesting on January 31, 2016
Additional disclosures
Vesting percent					38.00%
Vesting period					12 months
Restricted Stock, Inducement Awards, Tranche Two [Member] | Chief Executive Officer | Vesting on January 31, 2017
Additional disclosures
Vesting percent					41.00%
Restricted Stock, Inducement Awards, Tranche Two [Member] | Chief Executive Officer | Vesting on January 31, 2018
Additional disclosures
Vesting percent					21.00%
Phantom share awards
Total Number of Shares
Outstanding at the beginning of the period (in shares)				0	57,798
Granted (in shares)			0	0
Vested (in shares)					(57,798)
Outstanding at the end of the period (in shares)			0		0
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares					$ 12.24
Vested (in dollars per share) | $ / shares					$ 12.24
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture					57,798
Stock options
Employee and non-employee director stock and benefit plans
Exercise price (in dollars per share) | $ / shares			$ 16.18		$ 12.17
Additional disclosures
Share-based compensation expense | $			$ 1.1	$ 0.3	$ 0.3
Tax benefit related to share-based compensation expense | $			$ 0.0	0.0	$ 0.1
2007 Plan | Restricted share awards | Vesting on January 31, 2017
Additional disclosures
Vesting percent			33.33%
2007 Plan | Restricted share awards | Vesting on January 31, 2018
Additional disclosures
Vesting percent			66.67%
2007 Plan | Phantom share awards
Additional disclosures
Share-based compensation expense | $				0.1
Tax benefit related to share-based compensation expense | $				$ 0.1
Sale of subsidiaries | ITG Investment Research | Restricted share awards
Total Number of Shares
Vested (in shares)		(226,802)
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture		226,802
Cost to modify the vesting schedule | $		$ 0.7
Subsequent event | Stock options
Employee and non-employee director stock and benefit plans
Accelerated vesting	65,611
Exercise price (in dollars per share) | $ / shares	$ 16.18