Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2025 USD ($) security	Dec. 31, 2024 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 319,247	$ 1,568,486
12 Months or Greater - Fair Value	2,574,731	2,674,209
Total - Fair Value	2,893,978	4,242,695
Less Than 12 Months - Unrealized Losses	(7,182)	(23,848)
12 Months or Greater - Unrealized Losses	(222,097)	(322,308)
Total - Unrealized Losses	$ (229,279)	$ (346,156)
Number of securities in unrealized loss position (in securities) | security	747	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	$ 18,580	$ 37,017
12 Months or Greater - Fair Value	55,261	69,959
Total - Fair Value	73,841	106,976
Less Than 12 Months - Unrealized Losses	(35)	(437)
12 Months or Greater - Unrealized Losses	(2,778)	(4,650)
Total - Unrealized Losses	(2,813)	(5,087)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	114,628	409,406
12 Months or Greater - Fair Value	1,143,794	1,195,869
Total - Fair Value	1,258,422	1,605,275
Less Than 12 Months - Unrealized Losses	(5,946)	(5,621)
12 Months or Greater - Unrealized Losses	(134,328)	(187,168)
Total - Unrealized Losses	(140,274)	(192,789)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	144,911	852,752
12 Months or Greater - Fair Value	1,052,750	1,051,862
Total - Fair Value	1,197,661	1,904,614
Less Than 12 Months - Unrealized Losses	(247)	(10,334)
12 Months or Greater - Unrealized Losses	(63,906)	(102,505)
Total - Unrealized Losses	(64,153)	(112,839)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	26,582	20,090
12 Months or Greater - Fair Value	32,966	49,640
Total - Fair Value	59,548	69,730
Less Than 12 Months - Unrealized Losses	(889)	(184)
12 Months or Greater - Unrealized Losses	(465)	(1,050)
Total - Unrealized Losses	(1,354)	(1,234)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	3,925	171,654
12 Months or Greater - Fair Value	189,235	151,893
Total - Fair Value	193,160	323,547
Less Than 12 Months - Unrealized Losses	(2)	(5,498)
12 Months or Greater - Unrealized Losses	(17,802)	(20,030)
Total - Unrealized Losses	(17,804)	(25,528)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	3,614	77,567
12 Months or Greater - Fair Value	96,635	151,188
Total - Fair Value	100,249	228,755
Less Than 12 Months - Unrealized Losses	(40)	(1,774)
12 Months or Greater - Unrealized Losses	(2,421)	(6,216)
Total - Unrealized Losses	(2,461)	(7,990)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	7,007
12 Months or Greater - Fair Value	0
Total - Fair Value	7,007
Less Than 12 Months - Unrealized Losses	(23)
12 Months or Greater - Unrealized Losses	0
Total - Unrealized Losses	(23)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	4,090	3,798
Total - Fair Value	4,090	3,798
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(397)	(689)
Total - Unrealized Losses	$ (397)	$ (689)