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Fair value - Summary of Significant Inputs used for Fair Value Measurement of Fixed Maturity Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2025 | Dec. 31, 2024
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | $ 5,815,337 | $ 5,624,773
State and political subdivisions |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 476,141 | 467,476
Non-U.S. government |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 113,153 | 83,802
U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 2,900,205 | 2,825,679
Non-U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 768,953 | 772,624
Residential mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 111,683 | 8,364
Commercial mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 20,702 |
Other asset-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 1,135,492 | 1,189,465
Fair value, recurring |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 5,815,337 | 5,624,773
Fair value, recurring | U.S. government, agencies and GSEs |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 289,008 | 277,363
Fair value, recurring | State and political subdivisions |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 476,141 | 467,476
Fair value, recurring | Non-U.S. government |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 113,153 | 83,802
Fair value, recurring | U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 2,900,205 | 2,825,679
Fair value, recurring | Non-U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 768,953 | 772,624
Fair value, recurring | Residential mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 111,683 | 8,364
Fair value, recurring | Commercial mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 20,702 |
Fair value, recurring | Other asset-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 1,135,492 | 1,189,465
Level 2 | Fair value, recurring |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 5,535,633 | 5,343,232
Level 2 | Fair value, recurring | U.S. government, agencies and GSEs |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 289,008 | 277,363
Level 2 | Fair value, recurring | State and political subdivisions |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 476,141 | 467,476
Level 2 | Fair value, recurring | Non-U.S. government |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 113,153 | 83,802
Level 2 | Fair value, recurring | U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 2,674,235 | 2,602,893
Level 2 | Fair value, recurring | Non-U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 718,479 | 716,071
Level 2 | Fair value, recurring | Residential mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 111,683 | 8,364
Level 2 | Fair value, recurring | Commercial mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 20,702 |
Level 2 | Fair value, recurring | Other asset-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 1,132,232 | $ 1,187,263
Pricing Services | Level 2 | Fair value, recurring | U.S. government, agencies and GSEs |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 289,008 |
Pricing Services | Level 2 | Fair value, recurring | State and political subdivisions |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 476,141 |
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. government |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 113,153 |
Pricing Services | Level 2 | Fair value, recurring | U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 2,483,692 |
Pricing Services | Level 2 | Fair value, recurring | Non-U.S. corporate |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 644,100 |
Pricing Services | Level 2 | Fair value, recurring | Residential mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 111,683 |
Pricing Services | Level 2 | Fair value, recurring | Commercial mortgage-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | 20,702 |
Pricing Services | Level 2 | Fair value, recurring | Other asset-backed |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Fair value | $ 1,132,232 |

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