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Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024
Available For Sale Securities
Cost or Amortized Cost	$ 3,669,921	$ 3,642,362
Allowance for Credit Losses	(538)	(828)	$ (157)	$ (197)
Gross Unrealized Gains	15,518	32,237
Gross Unrealized Losses	(156,209)	(140,435)
Fair Value	3,528,692	3,533,336
U.S. government
Available For Sale Securities
Cost or Amortized Cost	305,863	331,233
Gross Unrealized Gains	2,333	4,909
Gross Unrealized Losses	(1,056)	(919)
Fair Value	307,140	335,223
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	27,280	37,379
Gross Unrealized Gains	264	677
Gross Unrealized Losses	(256)	(129)
Fair Value	27,288	37,927
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	16,882	13,831
Gross Unrealized Gains	102	274
Gross Unrealized Losses	(698)	(558)
Fair Value	16,286	13,547
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	629,029	634,349
Gross Unrealized Gains	2,587	4,718
Gross Unrealized Losses	(30,814)	(28,392)
Fair Value	600,802	610,675
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	726,648	685,126
Allowance for Credit Losses	(377)	(470)
Gross Unrealized Gains	1,684	3,640
Gross Unrealized Losses	(17,654)	(15,312)
Fair Value	710,301	672,984
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,535,290	1,502,843
Allowance for Credit Losses	(161)	(358)
Gross Unrealized Gains	7,861	16,951
Gross Unrealized Losses	(38,585)	(27,930)
Fair Value	1,504,405	1,491,506
Municipal
Available For Sale Securities
Cost or Amortized Cost	428,929	437,601
Gross Unrealized Gains	687	1,068
Gross Unrealized Losses	(67,146)	(67,195)
Fair Value	$ 362,470	$ 371,474

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