Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 530,885	$ 694,898
12 Months or Greater - Fair Value	2,885,956	3,664,661
Total - Fair Value	3,416,841	4,359,559
Less Than 12 Months - Unrealized Losses	(11,146)	(12,254)
12 Months or Greater - Unrealized Losses	(248,898)	(353,890)
Total - Unrealized Losses	$ (260,044)	$ (366,144)
Number of securities in unrealized loss position (in securities) | security	851	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	$ 11,859	$ 26,550
12 Months or Greater - Fair Value	78,089	98,359
Total - Fair Value	89,948	124,909
Less Than 12 Months - Unrealized Losses	(42)	(75)
12 Months or Greater - Unrealized Losses	(3,977)	(6,289)
Total - Unrealized Losses	(4,019)	(6,364)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	94,516	275,727
12 Months or Greater - Fair Value	1,235,787	1,200,533
Total - Fair Value	1,330,303	1,476,260
Less Than 12 Months - Unrealized Losses	(1,481)	(3,622)
12 Months or Greater - Unrealized Losses	(143,409)	(186,213)
Total - Unrealized Losses	(144,890)	(189,835)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	240,691	270,956
12 Months or Greater - Fair Value	1,152,482	1,604,021
Total - Fair Value	1,393,173	1,874,977
Less Than 12 Months - Unrealized Losses	(4,040)	(6,060)
12 Months or Greater - Unrealized Losses	(79,965)	(122,151)
Total - Unrealized Losses	(84,005)	(128,211)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	43,761	41,549
12 Months or Greater - Fair Value	26,459	62,611
Total - Fair Value	70,220	104,160
Less Than 12 Months - Unrealized Losses	(852)	(1,234)
12 Months or Greater - Unrealized Losses	(545)	(2,041)
Total - Unrealized Losses	(1,397)	(3,275)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	53,033	44,867
12 Months or Greater - Fair Value	168,955	176,349
Total - Fair Value	221,988	221,216
Less Than 12 Months - Unrealized Losses	(4,602)	(872)
12 Months or Greater - Unrealized Losses	(13,614)	(19,784)
Total - Unrealized Losses	(18,216)	(20,656)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	4,829	35,249
12 Months or Greater - Fair Value	220,234	244,216
Total - Fair Value	225,063	279,465
Less Than 12 Months - Unrealized Losses	(76)	(391)
12 Months or Greater - Unrealized Losses	(6,851)	(15,361)
Total - Unrealized Losses	(6,927)	(15,752)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	82,196	0
12 Months or Greater - Fair Value	0	274,729
Total - Fair Value	82,196	274,729
Less Than 12 Months - Unrealized Losses	(53)	0
12 Months or Greater - Unrealized Losses	0	(1,408)
Total - Unrealized Losses	(53)	(1,408)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	3,950	3,843
Total - Fair Value	3,950	3,843
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(537)	(643)
Total - Unrealized Losses	$ (537)	$ (643)