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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Feb. 24, 2022 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Loss on deconsolidation of russian entities and other charges | $ 52 |  |  |
Restricted cash |  | $ 348 | $ 303 |
Investment income (loss) |  | 21 | 61 | $ (22)
Capitalized computer software costs |  | 492 | 475 |
Accumulated amortization |  | 1,800 | 1,700 |
Liability for errors and omission, current and noncurrent |  | 419 | 434 |
Net uncollected premiums and claims and related payables |  | 13,000 | 13,000 |
Risk and Insurance Services |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Interest income, fiduciary assets |  | 120 | 15 | $ 46
Accounts Payable and Accrued Liabilities |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Liabilities for errors and omissions, current |  | $ 64 | $ 79 |
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 |  |
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 |  |
Office Space | Lease Obligations | Lease Concentration Risk |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Concentration risk (as a percent) |  | 99.00% |  |

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