Investments - Available-For-Sale Securities Disclosure (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	$ 7,983,271	$ 7,880,697
Allowance for Credit Losses	(30,381)	(28,212)	$ (33,554)	$ (45,721)
Unrealized Gains	41,627	50,076
Unrealized Losses	(411,013)	(403,364)
Fair Value	7,583,504	7,499,197
U.S. government and government agencies
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	161,335	223,157
Allowance for Credit Losses	0	0
Unrealized Gains	70	139
Unrealized Losses	(19,579)	(18,261)
Fair Value	141,826	205,035
Foreign government
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	10,703	11,140
Allowance for Credit Losses	(29)	(35)	(36)	(284)
Unrealized Gains	0	0
Unrealized Losses	(1,445)	(1,302)
Fair Value	9,229	9,803
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	570,438	612,938
Allowance for Credit Losses	(695)	(669)	(737)	(1,024)
Unrealized Gains	1,101	2,623
Unrealized Losses	(31,858)	(28,927)
Fair Value	538,986	585,965
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	2,930,310	2,834,048
Allowance for Credit Losses	(15,442)	(12,999)	(16,756)	(30,330)
Unrealized Gains	20,367	28,078
Unrealized Losses	(140,185)	(137,888)
Fair Value	2,795,050	2,711,239
Collateralized loan obligations ("CLO") and other asset-backed securities ("ABS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	1,958,551	1,911,831
Allowance for Credit Losses	(2,627)	(2,854)	(3,895)	(2,375)
Unrealized Gains	15,158	11,855
Unrealized Losses	(73,997)	(86,005)
Fair Value	1,897,085	1,834,827
Residential mortgage-backed securities ("RMBS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	1,623,948	1,568,960
Allowance for Credit Losses	(11,580)	(11,649)	(11,740)	(11,597)
Unrealized Gains	3,828	6,023
Unrealized Losses	(104,237)	(85,851)
Fair Value	1,511,959	1,477,483
Commercial mortgage-backed securities ("CMBS")
Debt Securities, Available-for-sale [Line Items]
Cost/ Amortized Cost	727,986	718,623
Allowance for Credit Losses	(8)	(6)	$ (390)	$ (111)
Unrealized Gains	1,103	1,358
Unrealized Losses	(39,712)	(45,130)
Fair Value	$ 689,369	$ 674,845