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Fair Value (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | $ 2,828,131 | $ 2,260,146
Total fixed maturities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 2,320,184 | 1,831,268
U.S. government treasuries |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 764,928 | 708,250
U.S. states, territories and municipalities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 13,467 | 4,370
Non-U.S. sovereign governments and supranationals |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 77,176 | 56,246
Corporate |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 1,133,797 | 863,876
Residential mortgage-backed securities - Agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 251,514 | 168,513
Residential mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 5,359 | 4,984
Commercial mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 33,109 | 10,423
Other asset-backed securities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 40,834 | 14,606
Short-term investments |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 507,947 | 428,878
Level 1 |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Total fixed maturities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | U.S. government treasuries |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | U.S. states, territories and municipalities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Non-U.S. sovereign governments and supranationals |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Corporate |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Residential mortgage-backed securities - Agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Residential mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Commercial mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Other asset-backed securities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 1 | Short-term investments |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 2 |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 2,828,131 | 2,260,146
Level 2 | Total fixed maturities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 2,320,184 | 1,831,268
Level 2 | U.S. government treasuries |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 764,928 | 708,250
Level 2 | U.S. states, territories and municipalities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 13,467 | 4,370
Level 2 | Non-U.S. sovereign governments and supranationals |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 77,176 | 56,246
Level 2 | Corporate |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 1,133,797 | 863,876
Level 2 | Residential mortgage-backed securities - Agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 251,514 | 168,513
Level 2 | Residential mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 5,359 | 4,984
Level 2 | Commercial mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 33,109 | 10,423
Level 2 | Other asset-backed securities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 40,834 | 14,606
Level 2 | Short-term investments |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 507,947 | 428,878
Level 3 |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Total fixed maturities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | U.S. government treasuries |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | U.S. states, territories and municipalities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Non-U.S. sovereign governments and supranationals |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Corporate |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Residential mortgage-backed securities - Agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Residential mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Commercial mortgage-backed securities - Non-agency |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Other asset-backed securities |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | 0 | 0
Level 3 | Short-term investments |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |
Investments, at fair value | $ 0 | $ 0

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