Summary of Significant Accounting Policies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2024 USD ($) segment	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of business segments | segment	2
Restricted cash, current	$ 455,000,000	$ 486,000,000
Equity method investment	3 months
Investment income	$ 12,000,000	5,000,000	$ 21,000,000
Indefinite lived identified intangible assets	0	0
Capitalized computer software costs	474,000,000	519,000,000
Accumulated amortization	2,100,000,000	2,000,000,000
Liability for errors and omission, current and noncurrent	391,000,000	385,000,000
Net uncollected premiums and claims and related payables	15,100,000,000	13,800,000,000
Mc Griff Insurance Services L L C
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase due to acquisition	465,000,000
Risk and Insurance Services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, fiduciary assets	497,000,000	453,000,000	$ 120,000,000
Accounts Payable and Accrued Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liabilities for errors and omissions, current	$ 86,000,000	$ 71,000,000
Minimum | Customer Lists
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life	10 years
Maximum | Customer Lists
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets, useful life	15 years
Furniture and equipment | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	3 years
Furniture and equipment | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	10 years
Building | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	30 years
Building | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	40 years
Office Space | Lease Obligations | Lease Concentration Risk
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk (as a percent)	98.00%
Software Development | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	3 years
Software Development | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	10 years