Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,562,347	$ 3,411,256
12 Months or Greater - Fair Value	3,914,277	1,592,040
Total - Fair Value	5,476,624	5,003,296
Less Than 12 Months - Unrealized Losses	(37,684)	(298,637)
12 Months or Greater - Unrealized Losses	(512,617)	(224,403)
Total - Unrealized Losses	$ (550,301)	$ (523,040)
Number of securities in unrealized loss position (in securities) | security	1,315	1,226
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 64,850	$ 67,531
12 Months or Greater - Fair Value	111,833	76,246
Total - Fair Value	176,683	143,777
Less Than 12 Months - Unrealized Losses	(538)	(3,583)
12 Months or Greater - Unrealized Losses	(9,227)	(6,100)
Total - Unrealized Losses	(9,765)	(9,683)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	674,000	1,344,272
12 Months or Greater - Fair Value	1,226,433	360,956
Total - Fair Value	1,900,433	1,705,228
Less Than 12 Months - Unrealized Losses	(14,743)	(157,903)
12 Months or Greater - Unrealized Losses	(259,414)	(98,170)
Total - Unrealized Losses	(274,157)	(256,073)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	600,174	1,488,255
12 Months or Greater - Fair Value	1,745,339	758,732
Total - Fair Value	2,345,513	2,246,987
Less Than 12 Months - Unrealized Losses	(12,938)	(109,976)
12 Months or Greater - Unrealized Losses	(187,712)	(86,401)
Total - Unrealized Losses	(200,650)	(196,377)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	74,002	53,201
12 Months or Greater - Fair Value	92,013	67,073
Total - Fair Value	166,015	120,274
Less Than 12 Months - Unrealized Losses	(950)	(1,008)
12 Months or Greater - Unrealized Losses	(4,101)	(5,033)
Total - Unrealized Losses	(5,051)	(6,041)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	116,145	77,563
12 Months or Greater - Fair Value	172,590	136,179
Total - Fair Value	288,735	213,742
Less Than 12 Months - Unrealized Losses	(6,082)	(8,572)
12 Months or Greater - Unrealized Losses	(28,645)	(17,172)
Total - Unrealized Losses	(34,727)	(25,744)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	33,176	166,973
12 Months or Greater - Fair Value	241,908	70,792
Total - Fair Value	275,084	237,765
Less Than 12 Months - Unrealized Losses	(2,433)	(12,951)
12 Months or Greater - Unrealized Losses	(20,403)	(7,640)
Total - Unrealized Losses	(22,836)	(20,591)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	213,461
12 Months or Greater - Fair Value	320,534	114,459
Total - Fair Value	320,534	327,920
Less Than 12 Months - Unrealized Losses	0	(4,644)
12 Months or Greater - Unrealized Losses	(2,256)	(3,185)
Total - Unrealized Losses	(2,256)	(7,829)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	3,627	3,787
Total - Fair Value	3,627	3,787
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(859)	(699)
Total - Unrealized Losses	$ (859)	(699)
Commercial paper
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value		0
12 Months or Greater - Fair Value		3,816
Total - Fair Value		3,816
Less Than 12 Months - Unrealized Losses		0
12 Months or Greater - Unrealized Losses		(3)
Total - Unrealized Losses		$ (3)