Restructuring Costs (Restructuring and Related Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Restructuring Cost and Reserve [Line Items]
Total	$ 46	$ 54	$ 96	$ 140	$ 276
Corporate
Restructuring Cost and Reserve [Line Items]
Total	8	18	13	37
Risk and Insurance Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Total	20	22	51	73
Consulting | Operating Segments
Restructuring Cost and Reserve [Line Items]
Total	$ 18	$ 14	$ 32	$ 30