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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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