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Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Available For Sale Securities
Cost or Amortized Cost	$ 3,573,679		$ 3,391,159
Allowance for Credit Losses	(245)	$ (157)	(197)	$ (228)	$ (237)	$ (306)
Gross Unrealized Gains	24,142		9,445
Gross Unrealized Losses	(176,815)		(224,611)
Fair Value	3,420,761		3,175,796
U.S. government
Available For Sale Securities
Cost or Amortized Cost	698,528		525,608
Gross Unrealized Gains	6,173		309
Gross Unrealized Losses	(2,629)		(10,282)
Fair Value	702,072		515,635
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	56,151		55,921
Gross Unrealized Gains	780		261
Gross Unrealized Losses	(933)		(1,844)
Fair Value	55,998		54,338
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	12,729		8,959
Gross Unrealized Gains	135
Gross Unrealized Losses	(776)		(1,088)
Fair Value	12,088		7,871
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	431,849		438,545
Gross Unrealized Gains	2,268		927
Gross Unrealized Losses	(35,569)		(43,249)
Fair Value	398,548		396,223
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	434,139		430,973
Allowance for Credit Losses	(77)		(8)
Gross Unrealized Gains	3,008		2,208
Gross Unrealized Losses	(18,681)		(22,925)
Fair Value	418,389		410,248
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,430,965		1,397,676
Allowance for Credit Losses	(168)		(189)
Gross Unrealized Gains	11,450		4,737
Gross Unrealized Losses	(36,320)		(55,703)
Fair Value	1,405,927		1,346,521
Municipal
Available For Sale Securities
Cost or Amortized Cost	509,318		533,477
Gross Unrealized Gains	328		1,003
Gross Unrealized Losses	(81,907)		(89,520)
Fair Value	$ 427,739		$ 444,960