Goodwill - Changes in Carrying Value of Goodwill by Reportable Segment (Details) $ in Millions	6 Months Ended
	Jun. 30, 2025 USD ($)
Goodwill [Line Items]
Beginning balance	$ 7,970
Goodwill of acquired businesses	137
Goodwill adjustment during measurement period	(5)
Goodwill disposed of relating to sales of businesses	(5)
Foreign currency translation adjustments during the year	268
Ending balance	8,365
Retail
Goodwill [Line Items]
Beginning balance	5,436
Goodwill of acquired businesses	21
Goodwill adjustment during measurement period	(5)
Goodwill disposed of relating to sales of businesses	(5)
Foreign currency translation adjustments during the year	218
Ending balance	5,665
Programs
Goodwill [Line Items]
Beginning balance	1,884
Goodwill of acquired businesses	24
Goodwill adjustment during measurement period	0
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	41
Ending balance	1,949
Wholesale Brokerage
Goodwill [Line Items]
Beginning balance	650
Goodwill of acquired businesses	92
Goodwill adjustment during measurement period	0
Goodwill disposed of relating to sales of businesses	0
Foreign currency translation adjustments during the year	9
Ending balance	$ 751