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Summary of Significant Accounting Principles and Practices - Additional Information (Details) £ in Millions, $ in Millions | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2024 USD ($) | Dec. 31, 2024 GBP (£) | Dec. 31, 2023 USD ($) | Dec. 31, 2023 GBP (£) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Accounting Policies [Abstract] |  |  |  |  |  |
Cash and cash equivalents and short-term investments | $ 1,300 |  | $ 1,100 |  |  |
Cash and cash equivalents and short term investments, period increase (decrease) | 157 |  |  |  |  |
Restricted cash and investments, current | 123 |  | 120 |  |  |
Operating funds required to be held by the Company in the U.K. | 79 | £ 63 | 80 | £ 63 |  |
Premium trust balances | 7,200 |  | 6,900 |  |  |
Fiduciary receivables | 10,300 |  | 9,400 |  |  |
Allowance for doubtful accounts | $ 75 |  | $ 79 |  | $ 76 | $ 90
VIE assets, percentage | 1.00% | 1.00% |  |  |  |

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