Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Available For Sale Securities
Cost or Amortized Cost	$ 3,655,954		$ 3,391,159
Allowance for Credit Losses	(899)	$ (245)	(197)	$ (234)	$ (228)	$ (306)
Gross Unrealized Gains	31,797		9,445
Gross Unrealized Losses	(150,368)		(224,611)
Fair Value	3,536,484		3,175,796
U.S. government
Available For Sale Securities
Cost or Amortized Cost	395,751		525,608
Gross Unrealized Gains	5,165		309
Gross Unrealized Losses	(1,323)		(10,282)
Fair Value	399,593		515,635
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	55,268		55,921
Gross Unrealized Gains	850		261
Gross Unrealized Losses	(635)		(1,844)
Fair Value	55,483		54,338
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	12,730		8,959
Gross Unrealized Gains	261
Gross Unrealized Losses	(614)		(1,088)
Fair Value	12,377		7,871
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	632,284		438,545
Gross Unrealized Gains	3,532		927
Gross Unrealized Losses	(31,465)		(43,249)
Fair Value	604,351		396,223
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	577,587		430,973
Allowance for Credit Losses	(614)		(8)
Gross Unrealized Gains	3,570		2,208
Gross Unrealized Losses	(16,789)		(22,925)
Fair Value	563,754		410,248
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,501,345		1,397,676
Allowance for Credit Losses	(285)		(189)
Gross Unrealized Gains	17,536		4,737
Gross Unrealized Losses	(28,561)		(55,703)
Fair Value	1,490,035		1,346,521
Municipal
Available For Sale Securities
Cost or Amortized Cost	480,989		533,477
Gross Unrealized Gains	883		1,003
Gross Unrealized Losses	(70,981)		(89,520)
Fair Value	$ 410,891		$ 444,960