Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Goodwill [Line Items]
Beginning balance	$ 4,736.8	$ 4,395.9
Goodwill of acquired businesses	1,994.5	350.0	$ 648.1
Goodwill disposed of relating to sales of businesses	(46.2)	(3.1)
Foreign currency translation adjustments during the year	(10.9)	(6.0)
Ending balance	6,674.2	4,736.8	4,395.9
Retail
Goodwill [Line Items]
Beginning balance	2,987.2	2,650.4
Goodwill of acquired businesses	1,366.6	346.0	300.0
Goodwill disposed of relating to sales of businesses	(38.5)	(3.1)
Foreign currency translation adjustments during the year	(6.3)	(6.1)
Ending balance	4,309.0	2,987.2	2,650.4
National Programs
Goodwill [Line Items]
Beginning balance	1,089.9	1,091.1
Goodwill of acquired businesses	516.5	(1.3)	163.1
Goodwill disposed of relating to sales of businesses	0.0	0.0
Foreign currency translation adjustments during the year	(4.0)	0.1
Ending balance	1,602.4	1,089.9	1,091.1
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	488.4	483.1
Goodwill of acquired businesses	111.4	5.3	185.0
Goodwill disposed of relating to sales of businesses	(7.7)	0.0
Foreign currency translation adjustments during the year	(0.6)	0.0
Ending balance	591.5	488.4	483.1
Services
Goodwill [Line Items]
Beginning balance	171.3	171.3
Goodwill of acquired businesses	0.0	0.0	0.1
Goodwill disposed of relating to sales of businesses	0.0	0.0
Foreign currency translation adjustments during the year	0.0	0.0
Ending balance	$ 171.3	$ 171.3	$ 171.3