Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	9 Months Ended
	Sep. 30, 2023 USD ($)
Goodwill [Line Items]
Beginning balance	$ 6,674.2
Goodwill adjustment during measurement period	(9.7)
Goodwill of acquired businesses	163.9
Goodwill disposed of relating to sales of businesses	(2.7)
Foreign currency translation adjustments during the year	19.9
Ending balance	6,845.6
Retail
Goodwill [Line Items]
Beginning balance	4,309.0
Goodwill adjustment during measurement period	15.5
Goodwill of acquired businesses	138.6
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	18.5
Ending balance	4,481.6
National Programs
Goodwill [Line Items]
Beginning balance	1,602.4
Goodwill adjustment during measurement period	(23.5)
Goodwill of acquired businesses	2.4
Goodwill disposed of relating to sales of businesses	(2.7)
Foreign currency translation adjustments during the year	(0.1)
Ending balance	1,578.5
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	591.5
Goodwill adjustment during measurement period	(1.7)
Goodwill of acquired businesses	22.9
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	1.5
Ending balance	614.2
Services
Goodwill [Line Items]
Beginning balance	171.3
Goodwill adjustment during measurement period	0.0
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