Goodwill and Acquired Client Relationships - Schedule of Changes in the Components of Acquired Client Relationships (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Definite-lived
Intangible amortization	$ (20.6)	$ (29.6)
Total
Beginning balance, net book value	1,182.0
Ending balance, net book value	1,133.8
Acquired Client Relationships
Definite-lived
Beginning balance, gross book value	1,248.8
Beginning balance, accumulated amortization	(1,039.0)
Beginning balance, net book value	209.8
Intangible impairments	0.0
Intangible amortization	(20.6)
Intangible amortization and impairments	(20.6)
Foreign currency translation, gross	(8.5)
Foreign currency translation, accumulated amortization	5.9
Foreign currency translation, net	(2.6)
Ending balance, gross book value	1,240.3
Ending balance, accumulated amortization	(1,053.7)
Ending balance, net book value	186.6
Indefinite-lived
Beginning balance, net book value	972.2
Intangible impairments	0.0
Foreign currency translation	(25.0)
Ending balance, net book value	947.2
Total
Beginning balance, net book value	1,182.0
Intangible amortization and impairments	(20.6)
Foreign currency translation	(27.6)
Ending balance, net book value	$ 1,133.8