Restructuring Costs (Restructuring and Related Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Restructuring Cost and Reserve [Line Items]
Total	$ 54	$ 52	$ 140	$ 170	$ 301
Corporate
Restructuring Cost and Reserve [Line Items]
Total	18	9	37	48
Risk and Insurance Services
Restructuring Cost and Reserve [Line Items]
Total	22	26	73	89
Consulting
Restructuring Cost and Reserve [Line Items]
Total	$ 14	$ 17	$ 30	$ 33