Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2025 USD ($) security	Dec. 31, 2024 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,181,659	$ 1,568,486
12 Months or Greater - Fair Value	2,657,772	2,674,209
Total - Fair Value	3,839,431	4,242,695
Less Than 12 Months - Unrealized Losses	(14,227)	(23,848)
12 Months or Greater - Unrealized Losses	(276,794)	(322,308)
Total - Unrealized Losses	$ (291,021)	$ (346,156)
Number of securities in unrealized loss position (in securities) | security	986	1,020
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 36,101	$ 37,017
12 Months or Greater - Fair Value	66,902	69,959
Total - Fair Value	103,003	106,976
Less Than 12 Months - Unrealized Losses	(63)	(437)
12 Months or Greater - Unrealized Losses	(3,749)	(4,650)
Total - Unrealized Losses	(3,812)	(5,087)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	378,830	409,406
12 Months or Greater - Fair Value	1,204,931	1,195,869
Total - Fair Value	1,583,761	1,605,275
Less Than 12 Months - Unrealized Losses	(6,953)	(5,621)
12 Months or Greater - Unrealized Losses	(158,166)	(187,168)
Total - Unrealized Losses	(165,119)	(192,789)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	595,473	852,752
12 Months or Greater - Fair Value	1,013,348	1,051,862
Total - Fair Value	1,608,821	1,904,614
Less Than 12 Months - Unrealized Losses	(5,842)	(10,334)
12 Months or Greater - Unrealized Losses	(87,623)	(102,505)
Total - Unrealized Losses	(93,465)	(112,839)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	9,237	20,090
12 Months or Greater - Fair Value	37,330	49,640
Total - Fair Value	46,567	69,730
Less Than 12 Months - Unrealized Losses	(94)	(184)
12 Months or Greater - Unrealized Losses	(821)	(1,050)
Total - Unrealized Losses	(915)	(1,234)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	31,034	171,654
12 Months or Greater - Fair Value	194,501	151,893
Total - Fair Value	225,535	323,547
Less Than 12 Months - Unrealized Losses	(379)	(5,498)
12 Months or Greater - Unrealized Losses	(21,141)	(20,030)
Total - Unrealized Losses	(21,520)	(25,528)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	81,237	77,567
12 Months or Greater - Fair Value	136,860	151,188
Total - Fair Value	218,097	228,755
Less Than 12 Months - Unrealized Losses	(864)	(1,774)
12 Months or Greater - Unrealized Losses	(4,707)	(6,216)
Total - Unrealized Losses	(5,571)	(7,990)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	49,747
12 Months or Greater - Fair Value	0
Total - Fair Value	49,747
Less Than 12 Months - Unrealized Losses	(32)
12 Months or Greater - Unrealized Losses	0
Total - Unrealized Losses	(32)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	3,900	3,798
Total - Fair Value	3,900	3,798
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(587)	(689)
Total - Unrealized Losses	$ (587)	$ (689)