Goodwill and Other Intangible Assets - Components of Other Intangible Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	$ 469.6	$ 467.2
Accumulated Amortization, total other intangible assets	(395.8)	(378.4)
Net Book Value, total other intangible assets	73.8	88.8
Client Relationships
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	413.4	410.2
Accumulated Amortization, total other intangible assets	(353.3)	(337.1)
Net Book Value, total other intangible assets	60.1	73.1
Trade Names
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	30.7	31.5
Accumulated Amortization, total other intangible assets	(17.0)	(15.8)
Net Book Value, total other intangible assets	13.7	15.7
Trademarks, Licenses and Agreements
Finite Lived Intangible Assets [Line Items]
Gross Carrying Amount, total other intangible assets	23.5	23.5
Accumulated Amortization, total other intangible assets	(23.5)	(23.5)
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)