Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Available For Sale Securities
Cost or Amortized Cost	$ 3,448,746	$ 3,391,159
Allowance for Credit Losses	(157)	(197)	$ (237)	$ (306)
Gross Unrealized Gains	15,323	9,445
Gross Unrealized Losses	(189,636)	(224,611)
Fair Value	3,274,276	3,175,796
U.S. government
Available For Sale Securities
Cost or Amortized Cost	560,814	525,608
Gross Unrealized Gains	3,092	309
Gross Unrealized Losses	(3,597)	(10,282)
Fair Value	560,309	515,635
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	56,036	55,921
Gross Unrealized Gains	632	261
Gross Unrealized Losses	(1,027)	(1,844)
Fair Value	55,641	54,338
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	12,728	8,959
Gross Unrealized Gains	75
Gross Unrealized Losses	(919)	(1,088)
Fair Value	11,884	7,871
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	437,829	438,545
Gross Unrealized Gains	2,125	927
Gross Unrealized Losses	(36,563)	(43,249)
Fair Value	403,391	396,223
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	436,159	430,973
Allowance for Credit Losses		(8)
Gross Unrealized Gains	2,671	2,208
Gross Unrealized Losses	(20,089)	(22,925)
Fair Value	418,741	410,248
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,424,850	1,397,676
Allowance for Credit Losses	(157)	(189)
Gross Unrealized Gains	6,207	4,737
Gross Unrealized Losses	(46,265)	(55,703)
Fair Value	1,384,635	1,346,521
Municipal
Available For Sale Securities
Cost or Amortized Cost	520,330	533,477
Gross Unrealized Gains	521	1,003
Gross Unrealized Losses	(81,176)	(89,520)
Fair Value	$ 439,675	$ 444,960