Goodwill - Changes in Carrying Value of Goodwill by Operating Segment (Details) $ in Millions	6 Months Ended
	Jun. 30, 2022 USD ($)
Goodwill [Line Items]
Beginning balance	$ 4,736.8
Goodwill of acquired businesses	421.4
Goodwill disposed of relating to sales of businesses	(2.0)
Foreign currency translation adjustments during the year	(7.2)
Ending balance	5,149.0
Retail
Goodwill [Line Items]
Beginning balance	2,987.2
Goodwill of acquired businesses	18.9
Goodwill disposed of relating to sales of businesses	(2.0)
Foreign currency translation adjustments during the year	(6.2)
Ending balance	2,997.9
National Programs
Goodwill [Line Items]
Beginning balance	1,089.9
Goodwill of acquired businesses	402.3
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	(1.0)
Ending balance	1,491.2
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	488.4
Goodwill of acquired businesses	0.2
Goodwill disposed of relating to sales of businesses	0.0
Foreign currency translation adjustments during the year	0.0
Ending balance	488.6
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