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Segment and Geographic Information - Revenues and Assets by Geographic Location (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Revenue	$ 604.1	[1]	$ 585.7	$ 570.9	$ 544.3	$ 550.3	$ 544.7	$ 554.1	$ 545.4	$ 2,305.0	$ 2,194.6	$ 2,484.5
Fixed Assets, Net	111.0					110.1				111.0	110.1	114.1
United States
Segment Reporting Information
Revenue										1,571.4	1,477.5	1,657.2
Fixed Assets, Net	97.8					97.3				97.8	97.3	98.6
United Kingdom
Segment Reporting Information
Revenue										587.3	566.4	645.3
Fixed Assets, Net	11.4					9.9				11.4	9.9	12.3
Other
Segment Reporting Information
Revenue										146.3	150.7	182.0
Fixed Assets, Net	$ 1.8					$ 2.9				$ 1.8	$ 2.9	$ 3.2
[1] In the fourth quarter of 2017, the Company recorded a provisional one-time net benefit from changes in U.S tax laws (see Note 22) and an expense associated with the impairment of one of its Affiliates accounted for under the equity method (see Note 13).

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Amount of fixed fee revenue for the management of an investment fund portfolio. Excludes investment advisory, distribution and servicing, and performance fees.

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