Restructuring Costs (Restructuring and Related Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Restructuring Cost and Reserve [Line Items]
Total	$ 44	$ 65	$ 86	$ 118	$ 301
Corporate
Restructuring Cost and Reserve [Line Items]
Total	10	27	19	39
Risk and Insurance Services
Restructuring Cost and Reserve [Line Items]
Total	29	31	51	63
Consulting
Restructuring Cost and Reserve [Line Items]
Total	$ 5	$ 7	$ 16	$ 16