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Goodwill and Acquired Client Relationships - Schedule of Changes in Acquired Client Relationships (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Book Value
Beginning balance, net book value	$ 1,449.7
Intangible amortization and impairments	(114.8)	$ (86.4)	$ (110.2)
Ending balance, net book value	1,309.9	1,449.7
Acquired Client Relationships (Net)
Definite-lived
Beginning balance, gross book value	1,295.5	1,290.0
Beginning balance, accumulated amortization	(874.5)	(788.1)
Beginning balance, net book value	421.0	501.9
Intangible amortization and impairments	(114.4)	(86.4)
Foreign currency translation	(3.0)	5.5
Ending balance, gross book value	1,292.5	1,295.5	1,290.0
Ending balance, accumulated amortization	(988.9)	(874.5)	(788.1)
Ending balance, net book value	303.6	421.0	501.9
Indefinite-lived
Beginning balance, net book value	1,028.7	995.5
Intangible amortization and impairments	(0.4)	0.0
Foreign currency translation	(22.0)	33.2
Ending balance, net book value	1,006.3	1,028.7	995.5
Net Book Value
Beginning balance, net book value	1,449.7	1,497.4
Intangible amortization and impairments	(114.8)	(86.4)
Foreign currency translation	(25.0)	38.7
Ending balance, net book value	$ 1,309.9	$ 1,449.7	$ 1,497.4

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