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Principles of Consolidation And Other Matters (Details) - USD ($) |  | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Jun. 10, 2021 | Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Revenue reduction for estimated commission revenue |  |  | $ 36,000,000 |  |  |
Operating funds related to regulatory requirements or as collateral under captive insurance arrangements |  | $ 304,000,000 |  | $ 304,000,000 |  | $ 270,000,000
Equity method investments lag period |  |  |  | 3 months |  |
Gain (loss) on investment income, net |  | $ 19,000,000 | $ (31,000,000) | $ 30,000,000 | $ (33,000,000) |
Charge related to revaluation of deferred tax assets and liabilities | $ 100,000,000 |  |  |  |  |
Effective tax rate (as a percent) |  | 31.60% | 26.20% | 27.90% | 24.90% |
Increase in effective tax rate due to change in U.K. corporate income tax rate, percent |  | 8.30% |  | 4.00% |  |
Unrecognized tax benefits |  | $ 94,000,000 |  | $ 94,000,000 |  | $ 98,000,000
Minimum |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Reasonably possible decrease in unrecognized tax benefits |  | 0 |  | 0 |  |
Maximum |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Reasonably possible decrease in unrecognized tax benefits |  | $ 33,000,000 |  | $ 33,000,000 |  |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Gain (loss) on investment income, net |  |  | $ (23,000,000) |  | $ (23,000,000) |

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