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Goodwill and Acquired Client Relationships - Schedule of Changes in Acquired Client Relationships (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Definite-lived
Foreign currency translation, accumulated depreciation			$ (3.7)	$ (5.5)
Net Book Value
Beginning balance, net book value			1,182.0
Intangible amortization and impairments			(140.5)	(144.5)	$ (114.8)
Ending balance, net book value			1,048.8	1,182.0
Acquired Client Relationships (Net)
Definite-lived
Beginning balance, gross book value			1,248.8	1,279.8
Beginning balance, accumulated amortization			(1,039.0)	(976.2)
Beginning balance, net book value			209.8	303.6
Intangible amortization and impairments			(69.8)	(93.4)
Foreign currency translation, gross book value			3.5	5.1
Foreign currency translation, net book value			(0.2)	(0.4)
Transfers, gross book value			(85.7)	(36.1)
Transfers, accumulated amortization			85.7	36.1
Ending balance, gross book value			1,166.6	1,248.8	1,279.8
Ending balance, accumulated amortization			(1,026.8)	(1,039.0)	(976.2)
Ending balance, net book value			139.8	209.8	303.6
Indefinite-lived
Beginning balance, net book value			972.2	1,006.3
Intangible impairments	$ (14.0)	$ (60.3)	(70.7)	(51.1)
Foreign currency translation			7.5	17.0
Ending balance, net book value			909.0	972.2	1,006.3
Net Book Value
Beginning balance, net book value			1,182.0	1,309.9
Intangible amortization and impairments			(140.5)	(144.5)
Foreign currency translation			7.3	16.6
Ending balance, net book value			$ 1,048.8	$ 1,182.0	$ 1,309.9

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