Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,576,386	$ 3,411,256
12 Months or Greater - Fair Value	3,640,778	1,592,040
Total - Fair Value	5,217,164	5,003,296
Less Than 12 Months - Unrealized Losses	(29,572)	(298,637)
12 Months or Greater - Unrealized Losses	(428,791)	(224,403)
Total - Unrealized Losses	$ (458,363)	$ (523,040)
Number of securities in unrealized loss position (in securities) | security	1,223	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	$ 76,161	$ 67,531
12 Months or Greater - Fair Value	93,067	76,246
Total - Fair Value	169,228	143,777
Less Than 12 Months - Unrealized Losses	(1,394)	(3,583)
12 Months or Greater - Unrealized Losses	(7,477)	(6,100)
Total - Unrealized Losses	(8,871)	(9,683)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	586,294	1,344,272
12 Months or Greater - Fair Value	1,077,509	360,956
Total - Fair Value	1,663,803	1,705,228
Less Than 12 Months - Unrealized Losses	(10,085)	(157,903)
12 Months or Greater - Unrealized Losses	(204,703)	(98,170)
Total - Unrealized Losses	(214,788)	(256,073)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	754,768	1,488,255
12 Months or Greater - Fair Value	1,639,146	758,732
Total - Fair Value	2,393,914	2,246,987
Less Than 12 Months - Unrealized Losses	(15,548)	(109,976)
12 Months or Greater - Unrealized Losses	(162,551)	(86,401)
Total - Unrealized Losses	(178,099)	(196,377)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	47,890	53,201
12 Months or Greater - Fair Value	74,887	67,073
Total - Fair Value	122,777	120,274
Less Than 12 Months - Unrealized Losses	(622)	(1,008)
12 Months or Greater - Unrealized Losses	(4,569)	(5,033)
Total - Unrealized Losses	(5,191)	(6,041)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	70,138	77,563
12 Months or Greater - Fair Value	182,408	136,179
Total - Fair Value	252,546	213,742
Less Than 12 Months - Unrealized Losses	(1,082)	(8,572)
12 Months or Greater - Unrealized Losses	(22,501)	(17,172)
Total - Unrealized Losses	(23,583)	(25,744)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	37,158	166,973
12 Months or Greater - Fair Value	243,457	70,792
Total - Fair Value	280,615	237,765
Less Than 12 Months - Unrealized Losses	(818)	(12,951)
12 Months or Greater - Unrealized Losses	(21,334)	(7,640)
Total - Unrealized Losses	(22,152)	(20,591)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	3,977	213,461
12 Months or Greater - Fair Value	324,562	114,459
Total - Fair Value	328,539	327,920
Less Than 12 Months - Unrealized Losses	(23)	(4,644)
12 Months or Greater - Unrealized Losses	(4,912)	(3,185)
Total - Unrealized Losses	(4,935)	(7,829)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	3,743	3,787
Total - Fair Value	3,743	3,787
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(743)	(699)
Total - Unrealized Losses	(743)	(699)
Commercial paper
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	1,999	3,816
Total - Fair Value	1,999	3,816
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(1)	(3)
Total - Unrealized Losses	$ (1)	$ (3)