Goodwill and Other Intangibles (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Goodwill, impairment loss	$ 0
Indefinite-lived intangible assets	0		$ 0		$ 0
Goodwill	18,235,000,000	$ 16,655,000,000	18,235,000,000	$ 16,655,000,000	$ 17,231,000,000	$ 16,251,000,000
Aggregate amortization expense	90,000,000	$ 85,000,000	269,000,000	$ 257,000,000
Risk and Insurance Services
Segment Reporting Information [Line Items]
Goodwill expected to be tax deductible	263,000,000		263,000,000
Goodwill	14,000,000,000.0		14,000,000,000.0
Consulting
Segment Reporting Information [Line Items]
Goodwill expected to be tax deductible	88,000,000		88,000,000
Goodwill	$ 4,200,000,000		$ 4,200,000,000