Investments - Securities In Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2022 USD ($) security	Dec. 31, 2021 USD ($) security
Schedule of Investments [Line Items]
Less than 12 months	$ 3,411,256	$ 1,970,111
12 months or greater	1,592,040	85,830
Total	5,003,296	2,055,941
Less than 12 months	(298,637)	(26,842)
12 months or greater	(224,403)	(1,625)
Total	$ (523,040)	$ (28,467)
Number of securities in unrealized loss position | security	1,226	610
U.S. government securities
Schedule of Investments [Line Items]
Less than 12 months	$ 67,531	$ 91,154
12 months or greater	76,246	2,616
Total	143,777	93,770
Less than 12 months	(3,583)	(790)
12 months or greater	(6,100)	(78)
Total	(9,683)	(868)
Obligations of U.S. states and political subdivisions
Schedule of Investments [Line Items]
Less than 12 months	1,344,272	452,021
12 months or greater	360,956	15,540
Total	1,705,228	467,561
Less than 12 months	(157,903)	(7,189)
12 months or greater	(98,170)	(207)
Total	(256,073)	(7,396)
Corporate debt securities
Schedule of Investments [Line Items]
Less than 12 months	1,488,255	865,085
12 months or greater	758,732	10,997
Total	2,246,987	876,082
Less than 12 months	(109,976)	(13,260)
12 months or greater	(86,401)	(516)
Total	(196,377)	(13,776)
ABS
Schedule of Investments [Line Items]
Less than 12 months	53,201	100,064
12 months or greater	67,073	1,552
Total	120,274	101,616
Less than 12 months	(1,008)	(998)
12 months or greater	(5,033)	(10)
Total	(6,041)	(1,008)
RMBS
Schedule of Investments [Line Items]
Less than 12 months	77,563	180,586
12 months or greater	136,179	31,641
Total	213,742	212,227
Less than 12 months	(8,572)	(2,548)
12 months or greater	(17,172)	(730)
Total	(25,744)	(3,278)
CMBS
Schedule of Investments [Line Items]
Less than 12 months	166,973	89,889
12 months or greater	70,792	1,511
Total	237,765	91,400
Less than 12 months	(12,951)	(1,887)
12 months or greater	(7,640)	(49)
Total	(20,591)	(1,936)
CLOs
Schedule of Investments [Line Items]
Less than 12 months	213,461	177,663
12 months or greater	114,459	21,973
Total	327,920	199,636
Less than 12 months	(4,644)	(71)
12 months or greater	(3,185)	(35)
Total	(7,829)	(106)
Foreign government debt
Schedule of Investments [Line Items]
Less than 12 months	0	13,649
12 months or greater	3,787	0
Total	3,787	13,649
Less than 12 months	0	(99)
12 months or greater	(699)	0
Total	(699)	$ (99)
Commercial paper
Schedule of Investments [Line Items]
Less than 12 months	0
12 months or greater	3,816
Total	3,816
Less than 12 months	0
12 months or greater	(3)
Total	$ (3)