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Investments - Schedule of Available-For-Sale Securities Disclosure (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	$ 9,409,353		$ 8,476,078
Allowance for Credit Losses	(29,076)	$ (28,733)	(31,948)	$ (28,504)	$ (31,309)	$ (28,212)
Unrealized Gains	133,587		53,285
Unrealized Losses	(238,440)		(370,081)
Fair Value	9,275,424		8,127,334
U.S. government and government agencies
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	158,593		139,906
Allowance for Credit Losses	0		0
Unrealized Gains	134		2
Unrealized Losses	(15,329)		(19,753)
Fair Value	143,398		120,155
Foreign government
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	10,784		10,656
Allowance for Credit Losses	(19)	(21)	(21)	(24)	(26)	(35)
Unrealized Gains	49		0
Unrealized Losses	(829)		(1,333)
Fair Value	9,985		9,302
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	580,895		483,609
Allowance for Credit Losses	(326)	(390)	(570)	(472)	(639)	(669)
Unrealized Gains	5,977		550
Unrealized Losses	(25,586)		(32,359)
Fair Value	560,960		451,230
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	3,366,977		3,181,046
Allowance for Credit Losses	(8,020)	(9,571)	(14,924)	(12,034)	(15,950)	(12,999)
Unrealized Gains	74,035		25,259
Unrealized Losses	(73,730)		(123,201)
Fair Value	3,359,262		3,068,180
Collateralized loan obligations ("CLO") and other asset-backed securities ("ABS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	2,495,028		2,065,611
Allowance for Credit Losses	(9,234)	(7,230)	(4,889)	(4,462)	(3,022)	(2,854)
Unrealized Gains	29,936		22,116
Unrealized Losses	(36,989)		(49,689)
Fair Value	2,478,741		2,033,149
Residential mortgage-backed securities ("RMBS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	2,060,837		1,812,744
Allowance for Credit Losses	(11,368)	(11,422)	(11,544)	(11,509)	(11,660)	(11,649)
Unrealized Gains	18,184		3,880
Unrealized Losses	(68,720)		(112,722)
Fair Value	1,998,933		1,692,358
Commercial mortgage-backed securities ("CMBS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	736,239		782,506
Allowance for Credit Losses	(109)	$ (99)	0	$ (3)	$ (12)	$ (6)
Unrealized Gains	5,272		1,478
Unrealized Losses	(17,257)		(31,024)
Fair Value	$ 724,145		$ 752,960