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Investments - Schedule of Fixed Maturity and Short-Term Investments (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | $ 2,812,412 | $ 2,294,936
Gross Unrealized Gains | 43,175 | 17,667
Gross Unrealized Losses | (27,456) | (52,457)
Investments, at fair value | 2,828,131 | 2,260,146
Total fixed maturities |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 2,306,168 | 1,867,499
Gross Unrealized Gains | 41,466 | 16,226
Gross Unrealized Losses | (27,450) | (52,457)
Investments, at fair value | 2,320,184 | 1,831,268
U.S. government treasuries |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 761,334 | 717,134
Gross Unrealized Gains | 9,273 | 5,137
Gross Unrealized Losses | (5,679) | (14,021)
Investments, at fair value | 764,928 | 708,250
U.S. states, territories and municipalities |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 13,528 | 4,656
Gross Unrealized Gains | 114 | 0
Gross Unrealized Losses | (175) | (286)
Investments, at fair value | 13,467 | 4,370
Non-U.S. sovereign governments and supranationals |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 74,679 | 55,662
Gross Unrealized Gains | 3,029 | 2,175
Gross Unrealized Losses | (532) | (1,591)
Investments, at fair value | 77,176 | 56,246
Corporate |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 1,118,605 | 877,493
Gross Unrealized Gains | 24,677 | 8,443
Gross Unrealized Losses | (9,485) | (22,060)
Investments, at fair value | 1,133,797 | 863,876
Residential mortgage-backed securities - Agency |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 258,788 | 180,661
Gross Unrealized Gains | 3,069 | 435
Gross Unrealized Losses | (10,343) | (12,583)
Investments, at fair value | 251,514 | 168,513
Residential mortgage-backed securities - Non-agency |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 5,790 | 5,639
Gross Unrealized Gains | 109 | 16
Gross Unrealized Losses | (540) | (671)
Investments, at fair value | 5,359 | 4,984
Commercial mortgage-backed securities - Non-agency |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 33,189 | 11,473
Gross Unrealized Gains | 535 | 0
Gross Unrealized Losses | (615) | (1,050)
Investments, at fair value | 33,109 | 10,423
Other asset-backed securities |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 40,255 | 14,781
Gross Unrealized Gains | 660 | 20
Gross Unrealized Losses | (81) | (195)
Investments, at fair value | 40,834 | 14,606
Short-term investments |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 506,244 | 427,437
Gross Unrealized Gains | 1,709 | 1,441
Gross Unrealized Losses | (6) | 0
Investments, at fair value | $ 507,947 | $ 428,878

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