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Transactions with Related Parties (Details) | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2023 USD ($) director | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2017 USD ($) | Jan. 01, 2024 | Jul. 01, 2021 | Jan. 01, 2021 USD ($) | Dec. 31, 2020 USD ($)
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |
Purchases of other investments | $ 2,171,000,000 | $ 1,720,000,000 | $ 1,859,000,000 |  |  |  |  |
Net reserves | 16,062,000,000 | 13,752,000,000 | 12,158,000,000 |  |  |  |  | $ 12,199,000,000
Reinsurance recoverable for paid and unpaid claims and claims adjustments | 7,064,000,000 | 6,564,000,000 | 5,881,000,000 |  |  |  |  |
Ceded premiums written | 4,935,000,000 | 4,249,000,000 | 3,735,000,000 |  |  |  |  |
Reinsurance balances payable | 2,000,000,000 | 1,530,000,000 |  |  |  |  |  |
Net premiums written | 13,468,000,000 | 11,078,000,000 | 9,017,000,000 |  |  |  |  |
Net premiums earned | $ 12,440,000,000 | 9,679,000,000 | $ 8,082,000,000 |  |  |  |  |
Forecast |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |
Put Call Right To Sell Purchase Shares |  |  |  |  | 33.30% |  |  |
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% |  |
Kelso And Company | Greysbridge Holdings Ltd |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |
Percentage ownership |  |  |  |  |  | 30.00% |  |
Warburg Pincus LLC | Greysbridge Holdings Ltd |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |
Percentage ownership |  |  |  |  |  | 30.00% |  |
Equity Method Investee | Premia Holdings Ltd |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |
Percentage ownership |  |  |  | 25.00% |  |  |  |
Purchases of other investments |  |  |  | $ 100,000,000 |  |  |  |
Number of directors | director | 2 |  |  |  |  |  |  |
Net reserves |  |  |  |  |  |  | $ 380,000,000 |
Reinsurance recoverable for paid and unpaid claims and claims adjustments | $ 0 | 0 |  |  |  |  |  |
Funds held liability | 0 | 0 |  |  |  |  |  |
Net premiums written | 80,000,000 | 121,000,000 |  |  |  |  |  |
Net premiums earned | 81,000,000 | 120,000,000 |  |  |  |  |  |
Funds Held under Reinsurance Agreements, Asset | 158,000,000 | 119,000,000 |  |  |  |  |  |
Equity Method Investee | Somers Holdings Ltd. |  |  |  |  |  |  |  |
Related Party Transaction [Line Items] |  |  |  |  |  |  |  |
Reinsurance recoverable for paid and unpaid claims and claims adjustments | 1,300,000,000 | 1,200,000,000 |  |  |  |  |  |
Reinsurance balances payable | 475,000,000 | 414,000,000 |  |  |  |  |  |
Senior notes | $ 35,000,000 |  |  |  |  |  |  |
Preferred shares, dividend rate (as a percent) | 6.50% |  |  |  |  |  |  |
Net premiums written | $ 574,000,000 | $ 519,000,000 |  |  |  |  |  |

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