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Transactions with Related Parties (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2024 USD ($) director | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2021
Related Party Transaction [Line Items] |  |  |  |  |
Purchases of other investments | $ 3,485 | $ 2,171 | $ 1,720 |  |
Net premiums written | 15,732 | 13,468 | 11,078 |  |
Net premiums earned | 15,100 | 12,440 | 9,679 |  |
Reinsurance recoverable for paid and unpaid claims and claims adjustments | 8,260 | 7,064 | $ 6,564 |  |
Reinsurance balances payable | 2,137 | 2,000 |  |  |
Other assets | 6,231 | 4,761 |  |  |
Other liabilities | $ 3,039 | 1,942 |  |  |
Premia Holdings Ltd |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Number of directors | director | 7 |  |  |  |
Greysbridge Holdings Ltd |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Percentage ownership |  |  |  |  | 40.00%
Equity Method Investee | Watford Insurance Company |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Consideration paid | $ 35 |  |  |  |
Equity Method Investee | Premia Holdings Ltd |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Percentage ownership |  |  |  | 25.00% |
Purchases of other investments |  |  |  | $ 100 |
Number of directors | director | 2 |  |  |  |
Net premiums written |  | 80 |  |  |
Net premiums earned |  | 81 |  |  |
Funds held asset | $ 137 | 158 |  |  |
Equity Method Investee | Greysbridge Holdings Ltd |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Other assets | 261 |  |  |  |
Other liabilities | 261 |  |  |  |
Equity Method Investee | Somers Holdings Ltd. |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |
Net premiums written | 738 | 574 |  |  |
Reinsurance recoverable for paid and unpaid claims and claims adjustments | 1,600 | 1,300 |  |  |
Reinsurance balances payable | 489 | $ 475 |  |  |
Senior notes | $ 35 |  |  |  |
Preferred shares, dividend rate (as a percent) | 6.50% |  |  |  |

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