Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	9 Months Ended
	Sep. 30, 2024 USD ($)
Goodwill [Line Items]
Beginning balance	$ 7,341
Goodwill of acquired businesses	120
Goodwill adjustment during measurement period	7
Foreign currency translation adjustments during the year	109
Ending balance	7,577
Retail
Goodwill [Line Items]
Beginning balance	4,870
Goodwill of acquired businesses	119
Goodwill adjustment during measurement period	(37)
Foreign currency translation adjustments during the year	80
Ending balance	5,032
Programs
Goodwill [Line Items]
Beginning balance	1,853
Goodwill of acquired businesses	0
Goodwill adjustment during measurement period	38
Foreign currency translation adjustments during the year	22
Ending balance	1,913
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	618
Goodwill of acquired businesses	1
Goodwill adjustment during measurement period	6
Foreign currency translation adjustments during the year	7
Ending balance	$ 632