Investments - Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Fair value
Less than 12 months	$ 109,976	$ 248,510
12 months and greater	1,762,055	1,939,305
Total Fair Value	1,872,031	2,187,815
Amortized Cost
Amortized cost, less than 12 months	111,037	250,863
12 months and greater	1,921,790	2,151,348
Total Cost or Amortized Cost	2,032,827	2,402,211
Unrealized Loss
Less than 12 months	(1,061)	(2,353)
12 months and greater	(159,735)	(212,043)
Total Unrealized Loss	(160,796)	(214,396)
U.S. government
Fair value
Less than 12 months		37,718
12 months and greater	174,088	204,556
Total Fair Value	174,088	242,274
Amortized Cost
Amortized cost, less than 12 months		37,950
12 months and greater	176,915	210,182
Total Cost or Amortized Cost	176,915	248,132
Unrealized Loss
Less than 12 months		(232)
12 months and greater	(2,827)	(5,626)
Total Unrealized Loss	(2,827)	(5,858)
U.S. Agency
Fair value
Less than 12 months	3,707	8,736
12 months and greater	29,421	29,632
Total Fair Value	33,128	38,368
Amortized Cost
Amortized cost, less than 12 months	3,712	8,790
12 months and greater	30,343	31,167
Total Cost or Amortized Cost	34,055	39,957
Unrealized Loss
Less than 12 months	(5)	(54)
12 months and greater	(922)	(1,535)
Total Unrealized Loss	(927)	(1,589)
Non-U.S. government & agency
Fair value
Less than 12 months	1,391
12 months and greater	4,044	3,882
Total Fair Value	5,435	3,882
Amortized Cost
Amortized cost, less than 12 months	1,394
12 months and greater	4,800	4,800
Total Cost or Amortized Cost	6,194	4,800
Unrealized Loss
Less than 12 months	(3)
12 months and greater	(756)	(918)
Total Unrealized Loss	(759)	(918)
Agency MBS
Fair value
Less than 12 months	18,667	61,196
12 months and greater	250,519	275,707
Total Fair Value	269,186	336,903
Amortized Cost
Amortized cost, less than 12 months	18,689	61,714
12 months and greater	280,473	313,091
Total Cost or Amortized Cost	299,162	374,805
Unrealized Loss
Less than 12 months	(22)	(518)
12 months and greater	(29,954)	(37,384)
Total Unrealized Loss	(29,976)	(37,902)
ABS/CMBS/MBS
Fair value
Less than 12 months	7,744	12,240
12 months and greater	177,139	211,436
Total Fair Value	184,883	223,676
Amortized Cost
Amortized cost, less than 12 months	7,757	12,367
12 months and greater	196,419	239,193
Total Cost or Amortized Cost	204,176	251,560
Unrealized Loss
Less than 12 months	(13)	(127)
12 months and greater	(19,280)	(27,757)
Total Unrealized Loss	(19,293)	(27,884)
Corporate
Fair value
Less than 12 months	58,742	67,402
12 months and greater	740,358	822,731
Total Fair Value	799,100	890,133
Amortized Cost
Amortized cost, less than 12 months	59,635	68,345
12 months and greater	775,181	878,419
Total Cost or Amortized Cost	834,816	946,764
Unrealized Loss
Less than 12 months	(893)	(943)
12 months and greater	(34,823)	(55,688)
Total Unrealized Loss	(35,716)	(56,631)
Municipal
Fair value
Less than 12 months	19,725	61,218
12 months and greater	386,486	391,361
Total Fair Value	406,211	452,579
Amortized Cost
Amortized cost, less than 12 months	19,850	61,697
12 months and greater	457,659	474,496
Total Cost or Amortized Cost	477,509	536,193
Unrealized Loss
Less than 12 months	(125)	(479)
12 months and greater	(71,173)	(83,135)
Total Unrealized Loss	$ (71,298)	$ (83,614)