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Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions |  | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 country segment | Dec. 31, 2021 USD ($) segment | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Number of business segments (segment) | segment |  | 2 |  |
Restricted cash |  | $ 303 | $ 270 |
Investment income (loss) |  | 61 | (22) | $ 22
Gains on equity securities |  | 5 | 27 | 10
Gain related to investments in private equity funds and other investments |  |  |  | 12
Capitalized computer software costs |  | 475 | 481 |
Accumulated amortization |  | 1,700 | 1,600 |
Liability for errors and omission, current and noncurrent |  | 434 | 639 |
Net uncollected premiums and claims and related payables |  | 13,000 | 11,200 |
Lease liability |  | 2,212 |  |
ROU asset |  | 1,868 | 1,894 |
Accounts Payable and Accrued Liabilities |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Liabilities for errors and omissions, current |  | $ 79 | 271 |
Office Space | Lease Obligations | Lease Concentration Risk |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Concentration risk (as a percent) |  | 99.00% |  |
Minimum | Customer Lists |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Intangible assets, useful life |  | 10 years |  |
Minimum | Furniture and equipment |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 3 years |  |
Minimum | Building |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 30 years |  |
Minimum | Software Development |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 3 years |  |
Maximum | Customer Lists |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Intangible assets, useful life |  | 15 years |  |
Maximum | Furniture and equipment |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 10 years |  |
Maximum | Building |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 40 years |  |
Maximum | Software Development |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Property, plant and equipment, useful life |  | 10 years |  |
JLT Transaction |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Number of business segments (segment) | segment | 3 |  |  |
Number of countries in which entity operates (country) | country | 41 |  |  |
Risk and insurance services |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Interest income, fiduciary assets |  | $ 15 | $ 46 | $ 105

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