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Income Taxes (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Nov. 30, 2021 | Dec. 31, 2020
Tax Credit Carryforward [Line Items] |  |  |  |  |
Undistributed earnings of foreign subsidiaries | $ 830 |  |  |  |
Differences related to non-U.S. operations | 586 | $ 543 |  |  |
Net increases in valuation allowances | 110 | 1 | $ 36 |  |
Change in valuation allowance | $ 94 | 5 | 2 |  |
Net operating loss carryforwards, subject to expiration (as a percent) | 10.00% |  |  |  |
Net operating loss carryforwards, not subject to expiration (as a percent) | 90.00% |  |  |  |
Unrecognized tax benefits | $ 122 | $ 94 | 87 |  |
Accrued interest and penalties, before any applicable federal benefit | 48 |  | $ 45 |  |
Maximum |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |
Reasonably possible decrease in unrecognized tax benefits | 58 |  |  |  |
Marsh India |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |
Percentage of interest acquired |  |  | 92.00% | 49.00% | 49.00%
State and Local |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |
Differences related to non-U.S. operations | 80 |  |  |  |
Operating loss carryforwards tax benefits | 4 |  |  |  |
Federal |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |
Operating loss carryforwards tax benefits | 1 |  |  |  |
Non-U.S. |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |
Operating loss carryforwards tax benefits | $ 363 |  |  |  |

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