Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) $ in Millions	3 Months Ended
Mar. 31, 2022 USD ($)
Goodwill [Line Items]
Beginning balance	$ 4,736.8
Goodwill of acquired businesses	404.6
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	(0.5)
Ending balance	5,140.9
Retail
Goodwill [Line Items]
Beginning balance	2,987.2
Goodwill of acquired businesses	2.5
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	(1.5)
Ending balance	2,988.2
National Programs
Goodwill [Line Items]
Beginning balance	1,089.9
Goodwill of acquired businesses	402.1
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	1.0
Ending balance	1,493.0
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	488.4
Goodwill of acquired businesses	0.0
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	0.0
Ending balance	488.4
Services
Goodwill [Line Items]
Beginning balance	171.3
Goodwill of acquired businesses	0.0
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	0.0
Ending balance	$ 171.3

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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