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	Mar. 31, 2023	Dec. 31, 2022
Fair value
Less than 12 months	$ 841,432	$ 1,619,637
12 months and greater	1,450,551	794,880
Total Fair Value	2,291,983	2,414,517
Amortized Cost
Amortized cost, less than 12 months	859,518	1,715,942
12 months and greater	1,668,139	978,797
Total Cost or Amortized Cost	2,527,657	2,694,739
Unrealized Loss
Less than 12 months	(18,086)	(96,305)
12 months and greater	(217,588)	(183,917)
Total Unrealized Loss	(235,674)	(280,222)
U.S. government
Fair value
Less than 12 months	258,557	399,361
12 months and greater	60,288	8,828
Total Fair Value	318,845	408,189
Amortized Cost
Amortized cost, less than 12 months	262,101	407,340
12 months and greater	63,011	9,720
Total Cost or Amortized Cost	325,112	417,060
Unrealized Loss
Less than 12 months	(3,544)	(7,979)
12 months and greater	(2,723)	(892)
Total Unrealized Loss	(6,267)	(8,871)
U.S. Agency
Fair value
Less than 12 months	26,616	32,987
12 months and greater	8,940	2,170
Total Fair Value	35,556	35,157
Amortized Cost
Amortized cost, less than 12 months	27,208	34,627
12 months and greater	9,979	2,567
Total Cost or Amortized Cost	37,187	37,194
Unrealized Loss
Less than 12 months	(592)	(1,640)
12 months and greater	(1,039)	(397)
Total Unrealized Loss	(1,631)	(2,037)
Non-U.S. government & agency
Fair value
Less than 12 months	2,631	3,626
12 months and greater	3,322	2,221
Total Fair Value	5,953	5,847
Amortized Cost
Amortized cost, less than 12 months	2,702	3,798
12 months and greater	4,097	3,000
Total Cost or Amortized Cost	6,799	6,798
Unrealized Loss
Less than 12 months	(71)	(172)
12 months and greater	(775)	(779)
Total Unrealized Loss	(846)	(951)
Agency MBS
Fair value
Less than 12 months	88,920	197,252
12 months and greater	227,228	117,851
Total Fair Value	316,148	315,103
Amortized Cost
Amortized cost, less than 12 months	91,195	212,776
12 months and greater	261,262	144,544
Total Cost or Amortized Cost	352,457	357,320
Unrealized Loss
Less than 12 months	(2,275)	(15,524)
12 months and greater	(34,034)	(26,693)
Total Unrealized Loss	(36,309)	(42,217)
ABS/CMBS/MBS
Fair value
Less than 12 months	25,033	96,754
12 months and greater	212,917	136,149
Total Fair Value	237,950	232,903
Amortized Cost
Amortized cost, less than 12 months	25,851	104,724
12 months and greater	244,164	163,623
Total Cost or Amortized Cost	270,015	268,347
Unrealized Loss
Less than 12 months	(818)	(7,970)
12 months and greater	(31,247)	(27,474)
Total Unrealized Loss	(32,065)	(35,444)
Corporate Debt
Fair value
Less than 12 months	383,271	660,830
12 months and greater	572,886	323,337
Total Fair Value	956,157	984,167
Amortized Cost
Amortized cost, less than 12 months	393,475	697,437
12 months and greater	637,012	374,707
Total Cost or Amortized Cost	1,030,487	1,072,144
Unrealized Loss
Less than 12 months	(10,204)	(36,607)
12 months and greater	(64,126)	(51,370)
Total Unrealized Loss	(74,330)	(87,977)
Municipal
Fair value
Less than 12 months	56,404	228,827
12 months and greater	364,970	204,324
Total Fair Value	421,374	433,151
Amortized Cost
Amortized cost, less than 12 months	56,986	255,240
12 months and greater	448,614	280,636
Total Cost or Amortized Cost	505,600	535,876
Unrealized Loss
Less than 12 months	(582)	(26,413)
12 months and greater	(83,644)	(76,312)
Total Unrealized Loss	$ (84,226)	$ (102,725)