Employee and Non-Employee Director Stock and Benefit Plans - Share Awards (Details) $ / shares in Units, $ in Millions			12 Months Ended
	May 27, 2016 USD ($) shares	Dec. 22, 2015 shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) installment item $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
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)			3,063,264	3,171,036	2,659,228
Granted (in shares)			1,259,392	1,665,916	2,196,191
Vested (in shares)			(1,331,021)	(1,042,238)	(1,159,047)
Forfeited (in shares)			(84,090)	(731,450)	(525,336)
Outstanding at the end of the period (in shares)			2,907,545	3,063,264	3,171,036
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 16.63	$ 16.75	$ 13.51
Granted (in dollars per share) | $ / shares			19.81	16.16	18.79
Vested (in dollars per share) | $ / shares			16.60	16.06	13.49
Forfeited (in dollars per share) | $ / shares			18.99	16.88	16.10
Outstanding at the end of the period (in dollars per share) | $ / shares			$ 18.10	$ 16.63	$ 16.75
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			1,331,021	1,042,238	1,159,047
Unrecognized compensation costs | $			$ 25.8
Weighted average period over which unrecognized compensation costs are expected to be recognized			1 year 3 months 18 days
Grant date fair value | $			$ 21.9
Tax benefits related to the vesting | $			1.5	$ 0.8	$ 2.9
Tax shortfalls | $			0.1	0.7	0.3
Share-based compensation expense | $			19.6	24.9	16.4
Tax benefit related to share-based compensation expense | $			$ 0.9	$ 8.9	$ 6.6
ROE-Based Market Based Or Performance Based
Total Number of Shares
Outstanding at the beginning of the period (in shares)			342,370	377,475	438,022
Granted (in shares)			140,796	228,895	263,766
Vested (in shares)			(51,498)	(73,243)	(97,112)
Forfeited (in shares)			(38,324)	(190,757)	(227,201)
Outstanding at the end of the period (in shares)			393,344	342,370	377,475
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			51,498	73,243	97,112
Performance-based share awards
Total Number of Shares
Outstanding at the end of the period (in shares)			140,796
Market-based share awards
Total Number of Shares
Outstanding at the end of the period (in shares)			45,065
Time-Based Restricted Stock Units
Total Number of Shares
Outstanding at the beginning of the period (in shares)			2,720,894	2,793,561	2,221,206
Granted (in shares)			1,118,596	1,437,021	1,932,425
Vested (in shares)			(1,279,523)	(968,995)	(1,061,935)
Forfeited (in shares)			(45,766)	(540,693)	(298,135)
Outstanding at the end of the period (in shares)			2,514,201	2,720,894	2,793,561
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			1,279,523	968,995	1,061,935
ROE-Based Restricted Stock Units
Total Number of Shares
Outstanding at the end of the period (in shares)			207,483
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	358,349
Granted (in shares)			0
Vested (in shares)				(57,798)	(257,279)
Forfeited (in shares)					(43,272)
Outstanding at the end of the period (in shares)			0	0	57,798
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares				$ 12.24	$ 11.61
Vested (in dollars per share) | $ / shares				$ 12.24	11.58
Forfeited (in dollars per share) | $ / shares					10.99
Outstanding at the end of the period (in dollars per share) | $ / shares					$ 12.24
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture				57,798	257,279
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	$ 2.2
Tax benefit related to share-based compensation expense | $				$ 0.1	0.8
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
Sale of subsidiaries | Energy research business | Restricted share awards
Total Number of Shares
Vested (in shares)		(113,718)
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture		113,718
Other revenue | Sale of subsidiaries | Energy research business
Additional disclosures
Cost to modify the vesting schedule | $					$ 1.4