Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($)
Goodwill [Line Items]
Beginning balance	$ 6,674.2
Goodwill of acquired businesses	23.0
Goodwill disposed of relating to sales of businesses	(2.7)
Foreign currency translation adjustments during the year	35.7
Ending balance	6,730.2
Retail
Goodwill [Line Items]
Beginning balance	4,309.0
Goodwill of acquired businesses	44.6
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	34.3
Ending balance	4,387.9
National Programs
Goodwill [Line Items]
Beginning balance	1,602.4
Goodwill of acquired businesses	(21.6)
Goodwill disposed of relating to sales of businesses	(2.7)
Foreign currency translation adjustments during the year	(1.1)
Ending balance	1,577.0
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	591.5
Goodwill of acquired businesses	0.0
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	2.5
Ending balance	594.0
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