Summary of Significant Accounting Policies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) segment	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of business segments (segment) | segment	2
Restricted cash	$ 486,000,000	$ 348,000,000
Investment income	5,000,000	21,000,000	$ 61,000,000
Indefinite lived identified intangible assets	0	0
Capitalized computer software costs	519,000,000	492,000,000
Accumulated amortization	2,000,000,000	1,800,000,000
Liability for errors and omission, current and noncurrent	385,000,000	419,000,000
Net uncollected premiums and claims and related payables	13,800,000,000	13,000,000,000
Total assets	48,030,000,000	44,114,000,000	44,010,000,000
Risk and Insurance Services
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, fiduciary assets	453,000,000	120,000,000	$ 15,000,000
Accounts Payable and Accrued Liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Liabilities for errors and omissions, current	$ 71,000,000	$ 64,000,000
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