Business Combinations - Additional Information (Details) $ in Millions		12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2024 USD ($) Acquisition	Dec. 31, 2023 USD ($) Acquisition	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Number of acquisitions | Acquisition		32	33
Measurement period adjustment, total considration		$ 11
Measurement period adjustment, fiduciary liabilities		20
Measurement period adjustment, amortizable intangible assets		11
Measurement period adjustment, other net asstes		9
Measurement period adjustment, goodwill		9
Cash paid	$ 4	934	$ 695
Measurement period adjustment, goodwill		9	(12)
Goodwill	707	707
Goodwill currently deductible for income tax purposes	121	121
Goodwill related to the recorded earn-out payables	35	35
Total revenues related to acquisitions		40
Income before income taxes related to acquisitions		1
Maximum future contingency payments acquisitions	497	497
Estimated acquisition earn-out payables	167	167	249	$ 252	$ 291
Goodwill currently non-deductible for income tax purposes excluding related to the recorded earn-out payables	551	551
Goodwill after deducted for income tax	586	586
Other Liabilities
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	92	92	103
Accounts payable
Business Acquisition [Line Items]
Estimated acquisition earn-out payables	75	75	146
Retail
Business Acquisition [Line Items]
Measurement period adjustment, goodwill		(37)	13
Goodwill net		634
Programs
Business Acquisition [Line Items]
Goodwill net		40
Wholesale Brokerage
Business Acquisition [Line Items]
Measurement period adjustment, goodwill		6	$ (2)
Goodwill net		$ 33
Assets and Assumed Certain Liabilities
Business Acquisition [Line Items]
Number of acquisitions | Acquisition		14
Business Combinations - Stock Deals
Business Acquisition [Line Items]
Number of acquisitions | Acquisition		14
Book of Business Purchases
Business Acquisition [Line Items]
Number of acquisitions | Acquisition		4
GRP Holdco Limited
Business Acquisition [Line Items]
Maximum potential earn- out payable	0	$ 0
Quintes Holding B.V | Retail
Business Acquisition [Line Items]
Goodwill	$ 468	$ 468