Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill [Line Items]
Beginning balance	$ 6,674.2	$ 4,736.8
Goodwill adjustment during measurement period	(12.0)
Goodwill of acquired businesses	620.7	1,994.5
Goodwill disposed of relating to sales of businesses	(46.9)	(46.2)
Foreign currency translation adjustments during the year	104.8	(10.9)
Ending balance	7,340.8	6,674.2
Retail
Goodwill [Line Items]
Beginning balance	4,309.0	2,987.2
Goodwill adjustment during measurement period	13.1
Goodwill of acquired businesses	335.4	1,366.6
Goodwill disposed of relating to sales of businesses	(0.2)	(38.5)
Foreign currency translation adjustments during the year	84.8	(6.3)
Ending balance	4,742.1	4,309.0
National Programs
Goodwill [Line Items]
Beginning balance	1,602.4	1,089.9
Goodwill adjustment during measurement period	(23.5)
Goodwill of acquired businesses	262.5	516.5
Goodwill disposed of relating to sales of businesses	(2.7)	0.0
Foreign currency translation adjustments during the year	14.3	(4.0)
Ending balance	1,853.0	1,602.4
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	591.5	488.4
Goodwill adjustment during measurement period	(1.6)
Goodwill of acquired businesses	22.8	111.4
Goodwill disposed of relating to sales of businesses	0.0	(7.7)
Foreign currency translation adjustments during the year	5.7	(0.6)
Ending balance	618.4	591.5
Services
Goodwill [Line Items]
Beginning balance	171.3	171.3
Goodwill adjustment during measurement period	0.0
Goodwill of acquired businesses	0.0	0.0
Goodwill disposed of relating to sales of businesses	(44.0)	0.0
Foreign currency translation adjustments during the year	0.0	0.0
Ending balance	$ 127.3	$ 171.3