Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	6 Months Ended
	Jun. 30, 2023 USD ($)
Goodwill [Line Items]
Beginning balance	$ 6,674.2
Goodwill adjustment during measurement period	(9.0)
Goodwill of acquired businesses	125.8
Goodwill disposed of relating to sales of businesses	(2.7)
Foreign currency translation adjustments during the year	76.8
Ending balance	6,865.1
Retail
Goodwill [Line Items]
Beginning balance	4,309.0
Goodwill adjustment during measurement period	14.9
Goodwill of acquired businesses	121.1
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	67.7
Ending balance	4,512.7
National Programs
Goodwill [Line Items]
Beginning balance	1,602.4
Goodwill adjustment during measurement period	(23.9)
Goodwill of acquired businesses	2.9
Goodwill disposed of relating to sales of businesses	(2.7)
Foreign currency translation adjustments during the year	3.9
Ending balance	1,582.6
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	591.5
Goodwill adjustment during measurement period	0.0
Goodwill of acquired businesses	1.8
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	5.2
Ending balance	598.5
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