Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2025 USD ($) security	Dec. 31, 2024 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 912,718	$ 1,568,486
12 Months or Greater - Fair Value	2,540,860	2,674,209
Total - Fair Value	3,453,578	4,242,695
Less Than 12 Months - Unrealized Losses	(12,881)	(23,848)
12 Months or Greater - Unrealized Losses	(253,859)	(322,308)
Total - Unrealized Losses	$ (266,740)	$ (346,156)
Number of securities in unrealized loss position (in securities) | security	899	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	$ 109,799	$ 37,017
12 Months or Greater - Fair Value	65,219	69,959
Total - Fair Value	175,018	106,976
Less Than 12 Months - Unrealized Losses	(109)	(437)
12 Months or Greater - Unrealized Losses	(3,343)	(4,650)
Total - Unrealized Losses	(3,452)	(5,087)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	258,879	409,406
12 Months or Greater - Fair Value	1,194,583	1,195,869
Total - Fair Value	1,453,462	1,605,275
Less Than 12 Months - Unrealized Losses	(8,787)	(5,621)
12 Months or Greater - Unrealized Losses	(152,827)	(187,168)
Total - Unrealized Losses	(161,614)	(192,789)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	439,548	852,752
12 Months or Greater - Fair Value	948,756	1,051,862
Total - Fair Value	1,388,304	1,904,614
Less Than 12 Months - Unrealized Losses	(3,104)	(10,334)
12 Months or Greater - Unrealized Losses	(72,994)	(102,505)
Total - Unrealized Losses	(76,098)	(112,839)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	6,207	20,090
12 Months or Greater - Fair Value	38,007	49,640
Total - Fair Value	44,214	69,730
Less Than 12 Months - Unrealized Losses	(39)	(184)
12 Months or Greater - Unrealized Losses	(657)	(1,050)
Total - Unrealized Losses	(696)	(1,234)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	29,666	171,654
12 Months or Greater - Fair Value	189,541	151,893
Total - Fair Value	219,207	323,547
Less Than 12 Months - Unrealized Losses	(320)	(5,498)
12 Months or Greater - Unrealized Losses	(20,322)	(20,030)
Total - Unrealized Losses	(20,642)	(25,528)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	68,619	77,567
12 Months or Greater - Fair Value	100,770	151,188
Total - Fair Value	169,389	228,755
Less Than 12 Months - Unrealized Losses	(522)	(1,774)
12 Months or Greater - Unrealized Losses	(3,213)	(6,216)
Total - Unrealized Losses	(3,735)	(7,990)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	3,984	3,798
Total - Fair Value	3,984	3,798
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(503)	(689)
Total - Unrealized Losses	$ (503)	$ (689)