Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	6 Months Ended
	Jun. 30, 2024 USD ($)
Goodwill [Line Items]
Beginning balance	$ 7,341
Goodwill of acquired businesses	95
Goodwill adjustment during measurement period	13
Foreign currency translation adjustments during the year	(18)
Ending balance	7,431
Retail
Goodwill [Line Items]
Beginning balance	4,870
Goodwill of acquired businesses	94
Goodwill adjustment during measurement period	6
Foreign currency translation adjustments during the year	(15)
Ending balance	4,955
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	1,851
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	0
Ending balance	$ 625