Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Available For Sale Securities
Cost or Amortized Cost	$ 3,391,159	$ 3,054,391
Allowance for Credit Losses	(197)	(306)	$ (339)
Gross Unrealized Gains	9,445	16,160
Gross Unrealized Losses	(224,611)	(214,396)
Fair Value	3,175,796	2,855,849
U.S. government
Available For Sale Securities
Cost or Amortized Cost	525,608	312,632
Gross Unrealized Gains	309	1,257
Gross Unrealized Losses	(10,282)	(5,858)
Fair Value	515,635	308,031
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	55,921	60,763
Gross Unrealized Gains	261	652
Gross Unrealized Losses	(1,844)	(1,589)
Fair Value	54,338	59,826
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	8,959	4,800
Gross Unrealized Losses	(1,088)	(918)
Fair Value	7,871	3,882
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	438,545	460,551
Gross Unrealized Gains	927	2,636
Gross Unrealized Losses	(43,249)	(37,902)
Fair Value	396,223	425,285
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	430,973	308,458
Allowance for Credit Losses	(8)	(3)
Gross Unrealized Gains	2,208	611
Gross Unrealized Losses	(22,925)	(27,884)
Fair Value	410,248	281,182
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,397,676	1,273,187
Allowance for Credit Losses	(189)	(303)
Gross Unrealized Gains	4,737	8,766
Gross Unrealized Losses	(55,703)	(56,631)
Fair Value	1,346,521	1,225,019
Municipal
Available For Sale Securities
Cost or Amortized Cost	533,477	634,000
Gross Unrealized Gains	1,003	2,238
Gross Unrealized Losses	(89,520)	(83,614)
Fair Value	$ 444,960	$ 552,624

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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