Employee and Non-Employee Director Stock and Benefit Plans - Share Awards (Details) $ / shares in Units, $ in Millions			12 Months Ended
	May 27, 2016 USD ($) shares	Dec. 22, 2015 shares	Dec. 31, 2016 USD ($) installment $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Restricted Stock Unit Awards
Total Number of Shares
Outstanding at the beginning of the period (in shares)			3,171,036	2,659,228	2,438,227
Granted (in shares)			1,665,916	2,196,191	1,477,389
Vested (in shares)			(1,042,238)	(1,159,047)	(1,123,720)
Forfeited (in shares)			(731,450)	(525,336)	(132,668)
Outstanding at the end of the period (in shares)			3,063,264	3,171,036	2,659,228
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 16.75	$ 13.51	$ 12.31
Granted (in dollars per share) | $ / shares			16.16	18.79	15.45
Vested (in dollars per share) | $ / shares			16.06	13.49	13.63
Forfeited (in dollars per share) | $ / shares			16.88	16.10	12.10
Outstanding at the end of the period (in dollars per share) | $ / shares			$ 16.63	$ 16.75	$ 13.51
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			1,042,238	1,159,047	1,123,720
Unrecognized compensation costs | $			$ 23.7
Weighted average period over which unrecognized compensation costs are expected to be recognized			1 year 9 months 18 days
Grant date fair value | $			$ 18.1
Tax benefits related to the vesting | $			0.8	$ 2.9	$ 1.3
Tax shortfalls | $			0.7	0.3	0.4
Share-based compensation expense | $			24.9	16.4	15.0
Tax benefit related to share-based compensation expense | $			$ 8.9	$ 6.6	$ 6.0
Performance-Based Restricted Stock Units
Total Number of Shares
Outstanding at the beginning of the period (in shares)			377,475	438,022	458,743
Granted (in shares)			228,895	263,766	219,644
Vested (in shares)			(73,243)	(97,112)	(145,786)
Forfeited (in shares)			(190,757)	(227,201)	(94,579)
Outstanding at the end of the period (in shares)			342,370	377,475	438,022
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			73,243	97,112	145,786
Market-Based Restricted Stock Units
Total Number of Shares
Outstanding at the end of the period (in shares)			96,563
Time-Based Restricted Stock Units
Total Number of Shares
Outstanding at the beginning of the period (in shares)			2,793,561	2,221,206	1,979,484
Granted (in shares)			1,437,021	1,932,425	1,257,745
Vested (in shares)			(968,995)	(1,061,935)	(977,934)
Forfeited (in shares)			(540,693)	(298,135)	(38,089)
Outstanding at the end of the period (in shares)			2,720,894	2,793,561	2,221,206
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			968,995	1,061,935	977,934
ROE-Based Restricted Stock Units
Total Number of Shares
Outstanding at the end of the period (in shares)			245,807
Restricted Stock, Inducement Awards, Tranche One [Member] | Chief Executive Officer
Total Number of Shares
Granted (in shares)			135,353
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
Total Number of Shares
Vesting percent			38.00%
Additional disclosures
Vesting period			12 months
Restricted Stock, Inducement Awards, Tranche Two [Member] | Chief Executive Officer | Vesting on January 31, 2017
Total Number of Shares
Vesting percent			41.00%
Restricted Stock, Inducement Awards, Tranche Two [Member] | Chief Executive Officer | Vesting on January 31, 2018
Total Number of Shares
Vesting percent			21.00%
Phantom share awards
Total Number of Shares
Outstanding at the beginning of the period (in shares)			57,798	358,349	671,762
Vested (in shares)			(57,798)	(257,279)	(310,121)
Forfeited (in shares)				(43,272)	(3,292)
Outstanding at the end of the period (in shares)			0	57,798	358,349
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares			$ 12.24	$ 11.61	$ 12.74
Vested (in dollars per share) | $ / shares			$ 12.24	11.58	14.06
Forfeited (in dollars per share) | $ / shares				10.99	11.73
Outstanding at the end of the period (in dollars per share) | $ / shares				$ 12.24	$ 11.61
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture			57,798	257,279	310,121
2007 Plan | Phantom share awards
Additional disclosures
Share-based compensation expense | $			$ 0.1	$ 2.2	$ 3.5
Tax benefit related to share-based compensation expense | $				0.8	$ 0.9
2007 Plan | Phantom share awards | Maximum
Additional disclosures
Tax benefit related to share-based compensation expense | $			$ 0.1
Sale of subsidiaries | ITG Investment Research | Restricted Stock Unit 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 Stock Unit 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