Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,370,910	$ 694,898
12 Months or Greater - Fair Value	3,299,189	3,664,661
Total - Fair Value	4,670,099	4,359,559
Less Than 12 Months - Unrealized Losses	(18,761)	(12,254)
12 Months or Greater - Unrealized Losses	(355,968)	(353,890)
Total - Unrealized Losses	$ (374,729)	$ (366,144)
Number of securities in unrealized loss position (in securities) | security	1,188	1,021
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 77,946	$ 26,550
12 Months or Greater - Fair Value	100,540	98,359
Total - Fair Value	178,486	124,909
Less Than 12 Months - Unrealized Losses	(159)	(75)
12 Months or Greater - Unrealized Losses	(6,152)	(6,289)
Total - Unrealized Losses	(6,311)	(6,364)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	433,539	275,727
12 Months or Greater - Fair Value	1,340,359	1,200,533
Total - Fair Value	1,773,898	1,476,260
Less Than 12 Months - Unrealized Losses	(3,733)	(3,622)
12 Months or Greater - Unrealized Losses	(193,300)	(186,213)
Total - Unrealized Losses	(197,033)	(189,835)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	704,646	270,956
12 Months or Greater - Fair Value	1,362,682	1,604,021
Total - Fair Value	2,067,328	1,874,977
Less Than 12 Months - Unrealized Losses	(8,593)	(6,060)
12 Months or Greater - Unrealized Losses	(121,621)	(122,151)
Total - Unrealized Losses	(130,214)	(128,211)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	53,203	41,549
12 Months or Greater - Fair Value	40,590	62,611
Total - Fair Value	93,793	104,160
Less Than 12 Months - Unrealized Losses	(987)	(1,234)
12 Months or Greater - Unrealized Losses	(1,033)	(2,041)
Total - Unrealized Losses	(2,020)	(3,275)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	78,939	44,867
12 Months or Greater - Fair Value	172,749	176,349
Total - Fair Value	251,688	221,216
Less Than 12 Months - Unrealized Losses	(4,990)	(872)
12 Months or Greater - Unrealized Losses	(20,410)	(19,784)
Total - Unrealized Losses	(25,400)	(20,656)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	2,767	35,249
12 Months or Greater - Fair Value	261,570	244,216
Total - Fair Value	264,337	279,465
Less Than 12 Months - Unrealized Losses	(25)	(391)
12 Months or Greater - Unrealized Losses	(12,734)	(15,361)
Total - Unrealized Losses	(12,759)	(15,752)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	19,870	0
12 Months or Greater - Fair Value	16,929	274,729
Total - Fair Value	36,799	274,729
Less Than 12 Months - Unrealized Losses	(274)	0
12 Months or Greater - Unrealized Losses	(1)	(1,408)
Total - Unrealized Losses	(275)	(1,408)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	3,770	3,843
Total - Fair Value	3,770	3,843
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(717)	(643)
Total - Unrealized Losses	$ (717)	$ (643)