Investments - Schedule of Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair value
Less than 12 months	$ 706,951	$ 1,619,637
12 months and greater	1,915,900	794,880
Total Fair Value	2,622,851	2,414,517
Amortized Cost
Amortized cost, less than 12 months	735,072	1,715,942
12 months and greater	2,217,342	978,797
Total Cost or Amortized Cost	2,952,414	2,694,739
Unrealized Loss
Less than 12 months	(28,121)	(96,305)
12 months and greater	(301,442)	(183,917)
Total Unrealized Loss	(329,563)	(280,222)
U.S. government
Fair value
Less than 12 months	82,683	399,361
12 months and greater	238,928	8,828
Total Fair Value	321,611	408,189
Amortized Cost
Amortized cost, less than 12 months	86,186	407,340
12 months and greater	248,464	9,720
Total Cost or Amortized Cost	334,650	417,060
Unrealized Loss
Less than 12 months	(3,503)	(7,979)
12 months and greater	(9,536)	(892)
Total Unrealized Loss	(13,039)	(8,871)
U.S. Agency
Fair value
Less than 12 months	15,658	32,987
12 months and greater	32,710	2,170
Total Fair Value	48,368	35,157
Amortized Cost
Amortized cost, less than 12 months	16,304	34,627
12 months and greater	35,173	2,567
Total Cost or Amortized Cost	51,477	37,194
Unrealized Loss
Less than 12 months	(646)	(1,640)
12 months and greater	(2,463)	(397)
Total Unrealized Loss	(3,109)	(2,037)
Non-U.S. government & agency
Fair value
Less than 12 months		3,626
12 months and greater	3,811	2,221
Total Fair Value	3,811	5,847
Amortized Cost
Amortized cost, less than 12 months		3,798
12 months and greater	4,800	3,000
Total Cost or Amortized Cost	4,800	6,798
Unrealized Loss
Less than 12 months		(172)
12 months and greater	(989)	(779)
Total Unrealized Loss	(989)	(951)
Agency MBS
Fair value
Less than 12 months	138,173	197,252
12 months and greater	260,503	117,851
Total Fair Value	398,676	315,103
Amortized Cost
Amortized cost, less than 12 months	143,662	212,776
12 months and greater	313,136	144,544
Total Cost or Amortized Cost	456,798	357,320
Unrealized Loss
Less than 12 months	(5,489)	(15,524)
12 months and greater	(52,633)	(26,693)
Total Unrealized Loss	(58,122)	(42,217)
ABS/CMBS/MBS
Fair value
Less than 12 months	41,219	96,754
12 months and greater	217,289	136,149
Total Fair Value	258,508	232,903
Amortized Cost
Amortized cost, less than 12 months	41,882	104,724
12 months and greater	253,567	163,623
Total Cost or Amortized Cost	295,449	268,347
Unrealized Loss
Less than 12 months	(663)	(7,970)
12 months and greater	(36,278)	(27,474)
Total Unrealized Loss	(36,941)	(35,444)
Corporate Debt
Fair value
Less than 12 months	274,290	660,830
12 months and greater	804,638	323,337
Total Fair Value	1,078,928	984,167
Amortized Cost
Amortized cost, less than 12 months	287,456	697,437
12 months and greater	891,492	374,707
Total Cost or Amortized Cost	1,178,948	1,072,144
Unrealized Loss
Less than 12 months	(13,166)	(36,607)
12 months and greater	(86,854)	(51,370)
Total Unrealized Loss	(100,020)	(87,977)
Municipal
Fair value
Less than 12 months	154,928	228,827
12 months and greater	358,021	204,324
Total Fair Value	512,949	433,151
Amortized Cost
Amortized cost, less than 12 months	159,582	255,240
12 months and greater	470,710	280,636
Total Cost or Amortized Cost	630,292	535,876
Unrealized Loss
Less than 12 months	(4,654)	(26,413)
12 months and greater	(112,689)	(76,312)
Total Unrealized Loss	$ (117,343)	$ (102,725)