Goodwill and Other Intangible Assets - Components of Other Intangible Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	$ 466.4	$ 471.3
Accumulated Amortization, total other intangible assets	(411.9)	(402.5)
Net Book Value, total other intangible assets	54.5	68.8
Client Relationships
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	412.7	417.0
Accumulated Amortization, total other intangible assets	(369.6)	(361.1)
Net Book Value, total other intangible assets	43.1	55.9
Trade Names
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	28.5	29.1
Accumulated Amortization, total other intangible assets	(17.1)	(16.2)
Net Book Value, total other intangible assets	11.4	12.9
Trademarks, Licenses and Agreements
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	23.2	23.2
Accumulated Amortization, total other intangible assets	(23.2)	(23.2)
Patents
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	2.0	2.0
Accumulated Amortization, total other intangible assets	$ (2.0)	$ (2.0)