Investments - Schedule of Available-For-Sale Securities Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	$ 8,080,498		$ 7,880,697
Allowance for Credit Losses	(31,309)	$ (30,381)	(28,212)	$ (31,425)	$ (33,554)	$ (45,721)
Unrealized Gains	38,756		50,076
Unrealized Losses	(418,912)		(403,364)
Fair Value	7,669,033		7,499,197
U.S. government and government agencies
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	170,819		223,157
Allowance for Credit Losses	0		0
Unrealized Gains	56		139
Unrealized Losses	(19,907)		(18,261)
Fair Value	150,968		205,035
Foreign government
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	10,687		11,140
Allowance for Credit Losses	(26)	(29)	(35)	(34)	(36)	(284)
Unrealized Gains	0		0
Unrealized Losses	(1,427)		(1,302)
Fair Value	9,234		9,803
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	558,455		612,938
Allowance for Credit Losses	(639)	(695)	(669)	(769)	(737)	(1,024)
Unrealized Gains	770		2,623
Unrealized Losses	(33,151)		(28,927)
Fair Value	525,435		585,965
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	2,987,824		2,834,048
Allowance for Credit Losses	(15,950)	(15,442)	(12,999)	(16,149)	(16,756)	(30,330)
Unrealized Gains	17,803		28,078
Unrealized Losses	(143,725)		(137,888)
Fair Value	2,845,952		2,711,239
Collateralized loan obligations ("CLO") and other asset-backed securities ("ABS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	1,972,687		1,911,831
Allowance for Credit Losses	(3,022)	(2,627)	(2,854)	(2,915)	(3,895)	(2,375)
Unrealized Gains	16,359		11,855
Unrealized Losses	(69,904)		(86,005)
Fair Value	1,916,120		1,834,827
Residential mortgage-backed securities ("RMBS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	1,624,078		1,568,960
Allowance for Credit Losses	(11,660)	(11,580)	(11,649)	(11,550)	(11,740)	(11,597)
Unrealized Gains	2,954		6,023
Unrealized Losses	(111,420)		(85,851)
Fair Value	1,503,952		1,477,483
Commercial mortgage-backed securities ("CMBS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	755,948		718,623
Allowance for Credit Losses	(12)	$ (8)	(6)	$ (8)	$ (390)	$ (111)
Unrealized Gains	814		1,358
Unrealized Losses	(39,378)		(45,130)
Fair Value	$ 717,372		$ 674,845