Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($)
Goodwill [Line Items]
Beginning balance	$ 7,341
Goodwill of acquired businesses	58
Goodwill adjustment during measurement period	8
Foreign currency translation adjustments during the year	(21)
Ending balance	7,386
Retail
Goodwill [Line Items]
Beginning balance	4,870
Goodwill of acquired businesses	58
Goodwill adjustment during measurement period	4
Foreign currency translation adjustments during the year	(18)
Ending balance	4,914
Programs
Goodwill [Line Items]
Beginning balance	1,853
Goodwill of acquired businesses	0
Goodwill adjustment during measurement period	(1)
Foreign currency translation adjustments during the year	(3)
Ending balance	1,849
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	618
Goodwill of acquired businesses	0
Goodwill adjustment during measurement period	5
Foreign currency translation adjustments during the year	0
Ending balance	$ 623