Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Segment Information (Details of Operating Segment Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting [Line Items]
Revenue	$ 6,863	$ 6,351	$ 21,864	$ 20,386
Operating Segments
Segment Reporting [Line Items]
Revenue	6,885	6,372	21,919	20,444
Operating Segments \| Risk and Insurance Services
Segment Reporting [Line Items]
Revenue	4,200	3,907	14,074	13,294
Operating Segments \| Risk and Insurance Services \| Risk
Segment Reporting [Line Items]
Revenue	3,796	3,472	11,709	10,907
Operating Segments \| Risk and Insurance Services \| Reinsurance
Segment Reporting [Line Items]
Revenue	404	435	2,365	2,387
Operating Segments \| Consulting
Segment Reporting [Line Items]
Revenue	2,685	2,465	7,845	7,150
Operating Segments \| Consulting \| People and Investments
Segment Reporting [Line Items]
Revenue	1,672	1,579	4,931	4,573
Operating Segments \| Consulting \| Management Consulting
Segment Reporting [Line Items]
Revenue	1,013	886	2,914	2,577
Corporate Eliminations
Segment Reporting [Line Items]
Revenue	$ (22)	$ (21)	$ (55)	$ (58)