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Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Available For Sale Securities
Cost or Amortized Cost	$ 3,892,806		$ 3,642,362
Allowance for Credit Losses	(522)	$ (463)	(828)	$ (899)	$ (245)	$ (197)
Gross Unrealized Gains	2,244		32,237
Gross Unrealized Losses	(293,559)		(140,435)
Fair Value	3,600,969		3,533,336
U.S. government
Available For Sale Securities
Cost or Amortized Cost	255,418		331,233
Gross Unrealized Gains			4,909
Gross Unrealized Losses	(9,445)		(919)
Fair Value	245,973		335,223
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	25,845		37,379
Gross Unrealized Gains	6		677
Gross Unrealized Losses	(964)		(129)
Fair Value	24,887		37,927
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	16,886		13,831
Gross Unrealized Gains			274
Gross Unrealized Losses	(1,255)		(558)
Fair Value	15,631		13,547
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	643,916		634,349
Gross Unrealized Gains	71		4,718
Gross Unrealized Losses	(63,742)		(28,392)
Fair Value	580,245		610,675
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	816,876		685,126
Allowance for Credit Losses	(197)		(470)
Gross Unrealized Gains	669		3,640
Gross Unrealized Losses	(35,453)		(15,312)
Fair Value	781,895		672,984
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,715,017		1,502,843
Allowance for Credit Losses	(325)		(358)
Gross Unrealized Gains	1,462		16,951
Gross Unrealized Losses	(92,200)		(27,930)
Fair Value	1,623,954		1,491,506
Municipal
Available For Sale Securities
Cost or Amortized Cost	418,848		437,601
Gross Unrealized Gains	36		1,068
Gross Unrealized Losses	(90,500)		(67,195)
Fair Value	$ 328,384		$ 371,474