Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	9 Months Ended
	Sep. 30, 2022 USD ($)
Goodwill [Line Items]
Beginning balance	$ 4,736.8
Goodwill of acquired businesses	1,922.2
Goodwill disposed of relating to sales of businesses	(2.6)
Foreign currency translation adjustments during the year	(134.1)
Ending balance	6,522.3
Retail
Goodwill [Line Items]
Beginning balance	2,987.2
Goodwill of acquired businesses	1,389.0
Goodwill disposed of relating to sales of businesses	(2.6)
Foreign currency translation adjustments during the year	(118.1)
Ending balance	4,255.5
National Programs
Goodwill [Line Items]
Beginning balance	1,089.9
Goodwill of acquired businesses	427.8
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	(6.9)
Ending balance	1,510.8
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	488.4
Goodwill of acquired businesses	105.4
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	(9.1)
Ending balance	584.7
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