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	Jun. 30, 2023	Dec. 31, 2022
Fair value
Less than 12 months	$ 977,565	$ 1,619,637
12 months and greater	1,515,435	794,880
Total Fair Value	2,493,000	2,414,517
Amortized Cost
Amortized cost, less than 12 months	1,004,025	1,715,942
12 months and greater	1,749,914	978,797
Total Cost or Amortized Cost	2,753,939	2,694,739
Unrealized Loss
Less than 12 months	(26,460)	(96,305)
12 months and greater	(234,479)	(183,917)
Total Unrealized Loss	(260,939)	(280,222)
U.S. government
Fair value
Less than 12 months	220,227	399,361
12 months and greater	95,748	8,828
Total Fair Value	315,975	408,189
Amortized Cost
Amortized cost, less than 12 months	225,987	407,340
12 months and greater	99,905	9,720
Total Cost or Amortized Cost	325,892	417,060
Unrealized Loss
Less than 12 months	(5,760)	(7,979)
12 months and greater	(4,157)	(892)
Total Unrealized Loss	(9,917)	(8,871)
U.S. Agency
Fair value
Less than 12 months	33,951	32,987
12 months and greater	9,768	2,170
Total Fair Value	43,719	35,157
Amortized Cost
Amortized cost, less than 12 months	34,975	34,627
12 months and greater	10,962	2,567
Total Cost or Amortized Cost	45,937	37,194
Unrealized Loss
Less than 12 months	(1,024)	(1,640)
12 months and greater	(1,194)	(397)
Total Unrealized Loss	(2,218)	(2,037)
Non-U.S. government & agency
Fair value
Less than 12 months	658	3,626
12 months and greater	3,302	2,221
Total Fair Value	3,960	5,847
Amortized Cost
Amortized cost, less than 12 months	702	3,798
12 months and greater	4,097	3,000
Total Cost or Amortized Cost	4,799	6,798
Unrealized Loss
Less than 12 months	(44)	(172)
12 months and greater	(795)	(779)
Total Unrealized Loss	(839)	(951)
Agency MBS
Fair value
Less than 12 months	141,545	197,252
12 months and greater	216,878	117,851
Total Fair Value	358,423	315,103
Amortized Cost
Amortized cost, less than 12 months	146,150	212,776
12 months and greater	254,952	144,544
Total Cost or Amortized Cost	401,102	357,320
Unrealized Loss
Less than 12 months	(4,605)	(15,524)
12 months and greater	(38,074)	(26,693)
Total Unrealized Loss	(42,679)	(42,217)
ABS/CMBS/MBS
Fair value
Less than 12 months	20,483	96,754
12 months and greater	219,639	136,149
Total Fair Value	240,122	232,903
Amortized Cost
Amortized cost, less than 12 months	21,028	104,724
12 months and greater	252,692	163,623
Total Cost or Amortized Cost	273,720	268,347
Unrealized Loss
Less than 12 months	(545)	(7,970)
12 months and greater	(33,053)	(27,474)
Total Unrealized Loss	(33,598)	(35,444)
Corporate Debt
Fair value
Less than 12 months	429,215	660,830
12 months and greater	607,324	323,337
Total Fair Value	1,036,539	984,167
Amortized Cost
Amortized cost, less than 12 months	441,843	697,437
12 months and greater	676,263	374,707
Total Cost or Amortized Cost	1,118,106	1,072,144
Unrealized Loss
Less than 12 months	(12,628)	(36,607)
12 months and greater	(68,939)	(51,370)
Total Unrealized Loss	(81,567)	(87,977)
Municipal
Fair value
Less than 12 months	131,486	228,827
12 months and greater	362,776	204,324
Total Fair Value	494,262	433,151
Amortized Cost
Amortized cost, less than 12 months	133,340	255,240
12 months and greater	451,043	280,636
Total Cost or Amortized Cost	584,383	535,876
Unrealized Loss
Less than 12 months	(1,854)	(26,413)
12 months and greater	(88,267)	(76,312)
Total Unrealized Loss	$ (90,121)	$ (102,725)