Restructuring Costs (Restructuring and Related Costs) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Restructuring Cost and Reserve [Line Items]
Total	$ 32	$ 42	$ 276
Corporate
Restructuring Cost and Reserve [Line Items]
Total	1	9
Risk and Insurance Services | Operating Segments
Restructuring Cost and Reserve [Line Items]
Total	23	22
Consulting | Operating Segments
Restructuring Cost and Reserve [Line Items]
Total	$ 8	$ 11