Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	$ 1,634,202	$ 3,411,256
12 Months or Greater - Fair Value	3,143,669	1,592,040
Total - Fair Value	4,777,871	5,003,296
Less Than 12 Months - Unrealized Losses	(44,345)	(298,637)
12 Months or Greater - Unrealized Losses	(382,232)	(224,403)
Total - Unrealized Losses	$ (426,577)	$ (523,040)
Number of securities in unrealized loss position (in securities) | security	1,152	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	$ 59,935	$ 67,531
12 Months or Greater - Fair Value	84,568	76,246
Total - Fair Value	144,503	143,777
Less Than 12 Months - Unrealized Losses	(1,108)	(3,583)
12 Months or Greater - Unrealized Losses	(6,605)	(6,100)
Total - Unrealized Losses	(7,713)	(9,683)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	615,219	1,344,272
12 Months or Greater - Fair Value	962,757	360,956
Total - Fair Value	1,577,976	1,705,228
Less Than 12 Months - Unrealized Losses	(13,633)	(157,903)
12 Months or Greater - Unrealized Losses	(187,406)	(98,170)
Total - Unrealized Losses	(201,039)	(256,073)
Corporate debt securities
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	889,684	1,488,255
12 Months or Greater - Fair Value	1,269,912	758,732
Total - Fair Value	2,159,596	2,246,987
Less Than 12 Months - Unrealized Losses	(28,273)	(109,976)
12 Months or Greater - Unrealized Losses	(136,652)	(86,401)
Total - Unrealized Losses	(164,925)	(196,377)
ABS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	36,028	53,201
12 Months or Greater - Fair Value	77,206	67,073
Total - Fair Value	113,234	120,274
Less Than 12 Months - Unrealized Losses	(245)	(1,008)
12 Months or Greater - Unrealized Losses	(4,574)	(5,033)
Total - Unrealized Losses	(4,819)	(6,041)
RMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	4,078	77,563
12 Months or Greater - Fair Value	207,258	136,179
Total - Fair Value	211,336	213,742
Less Than 12 Months - Unrealized Losses	(177)	(8,572)
12 Months or Greater - Unrealized Losses	(22,079)	(17,172)
Total - Unrealized Losses	(22,256)	(25,744)
CMBS
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	11,699	166,973
12 Months or Greater - Fair Value	221,259	70,792
Total - Fair Value	232,958	237,765
Less Than 12 Months - Unrealized Losses	(714)	(12,951)
12 Months or Greater - Unrealized Losses	(17,990)	(7,640)
Total - Unrealized Losses	(18,704)	(20,591)
CLOs
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	17,068	213,461
12 Months or Greater - Fair Value	313,506	114,459
Total - Fair Value	330,574	327,920
Less Than 12 Months - Unrealized Losses	(195)	(4,644)
12 Months or Greater - Unrealized Losses	(6,264)	(3,185)
Total - Unrealized Losses	(6,459)	(7,829)
Foreign government debt
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	0	0
12 Months or Greater - Fair Value	3,828	3,787
Total - Fair Value	3,828	3,787
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(659)	(699)
Total - Unrealized Losses	(659)	(699)
Commercial paper
Schedule of Investments [Line Items]
Less Than 12 Months - Fair Value	491	0
12 Months or Greater - Fair Value	3,375	3,816
Total - Fair Value	3,866	3,816
Less Than 12 Months - Unrealized Losses	0	0
12 Months or Greater - Unrealized Losses	(3)	(3)
Total - Unrealized Losses	$ (3)	$ (3)