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Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Mar. 31, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024
Available For Sale Securities
Cost or Amortized Cost	$ 3,760,876		$ 3,642,362
Allowance for Credit Losses	(463)	$ (538)	(828)	$ (245)	$ (157)	$ (197)
Gross Unrealized Gains	13,680		32,237
Gross Unrealized Losses	(156,151)		(140,435)
Fair Value	3,617,942		3,533,336
U.S. government
Available For Sale Securities
Cost or Amortized Cost	282,536		331,233
Gross Unrealized Gains	735		4,909
Gross Unrealized Losses	(1,564)		(919)
Fair Value	281,707		335,223
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	26,313		37,379
Gross Unrealized Gains	163		677
Gross Unrealized Losses	(282)		(129)
Fair Value	26,194		37,927
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	16,884		13,831
Gross Unrealized Gains	157		274
Gross Unrealized Losses	(570)		(558)
Fair Value	16,471		13,547
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	625,626		634,349
Gross Unrealized Gains	1,714		4,718
Gross Unrealized Losses	(33,005)		(28,392)
Fair Value	594,335		610,675
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	775,674		685,126
Allowance for Credit Losses	(288)		(470)
Gross Unrealized Gains	1,549		3,640
Gross Unrealized Losses	(18,903)		(15,312)
Fair Value	758,032		672,984
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,609,863		1,502,843
Allowance for Credit Losses	(175)		(358)
Gross Unrealized Gains	8,176		16,951
Gross Unrealized Losses	(35,321)		(27,930)
Fair Value	1,582,543		1,491,506
Municipal
Available For Sale Securities
Cost or Amortized Cost	423,980		437,601
Gross Unrealized Gains	1,186		1,068
Gross Unrealized Losses	(66,506)		(67,195)
Fair Value	$ 358,660		$ 371,474