Investments - Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Fair value
Less than 12 months	$ 909,943	$ 248,510
12 months and greater	1,633,651	1,939,305
Total Fair Value	2,543,594	2,187,815
Amortized Cost
Amortized cost, less than 12 months	932,763	250,863
12 months and greater	1,835,442	2,151,348
Total Cost or Amortized Cost	2,768,205	2,402,211
Unrealized Loss
Less than 12 months	(22,820)	(2,353)
12 months and greater	(201,791)	(212,043)
Total Unrealized Loss	(224,611)	(214,396)
U.S. government
Fair value
Less than 12 months	303,226	37,718
12 months and greater	157,418	204,556
Total Fair Value	460,644	242,274
Amortized Cost
Amortized cost, less than 12 months	309,836	37,950
12 months and greater	161,090	210,182
Total Cost or Amortized Cost	470,926	248,132
Unrealized Loss
Less than 12 months	(6,610)	(232)
12 months and greater	(3,672)	(5,626)
Total Unrealized Loss	(10,282)	(5,858)
U.S. Agency
Fair value
Less than 12 months	24,024	8,736
12 months and greater	18,330	29,632
Total Fair Value	42,354	38,368
Amortized Cost
Amortized cost, less than 12 months	24,910	8,790
12 months and greater	19,288	31,167
Total Cost or Amortized Cost	44,198	39,957
Unrealized Loss
Less than 12 months	(886)	(54)
12 months and greater	(958)	(1,535)
Total Unrealized Loss	(1,844)	(1,589)
Non-U.S. government & agency
Fair value
Less than 12 months	4,075
12 months and greater	3,796	3,882
Total Fair Value	7,871	3,882
Amortized Cost
Amortized cost, less than 12 months	4,158
12 months and greater	4,801	4,800
Total Cost or Amortized Cost	8,959	4,800
Unrealized Loss
Less than 12 months	(83)
12 months and greater	(1,005)	(918)
Total Unrealized Loss	(1,088)	(918)
Agency MBS
Fair value
Less than 12 months	108,772	61,196
12 months and greater	233,625	275,707
Total Fair Value	342,397	336,903
Amortized Cost
Amortized cost, less than 12 months	111,674	61,714
12 months and greater	273,972	313,091
Total Cost or Amortized Cost	385,646	374,805
Unrealized Loss
Less than 12 months	(2,902)	(518)
12 months and greater	(40,347)	(37,384)
Total Unrealized Loss	(43,249)	(37,902)
ABS/CMBS/MBS
Fair value
Less than 12 months	43,027	12,240
12 months and greater	164,433	211,436
Total Fair Value	207,460	223,676
Amortized Cost
Amortized cost, less than 12 months	43,395	12,367
12 months and greater	186,990	239,193
Total Cost or Amortized Cost	230,385	251,560
Unrealized Loss
Less than 12 months	(368)	(127)
12 months and greater	(22,557)	(27,757)
Total Unrealized Loss	(22,925)	(27,884)
Corporate
Fair value
Less than 12 months	378,305	67,402
12 months and greater	700,574	822,731
Total Fair Value	1,078,879	890,133
Amortized Cost
Amortized cost, less than 12 months	389,299	68,345
12 months and greater	745,283	878,419
Total Cost or Amortized Cost	1,134,582	946,764
Unrealized Loss
Less than 12 months	(10,994)	(943)
12 months and greater	(44,709)	(55,688)
Total Unrealized Loss	(55,703)	(56,631)
Municipal
Fair value
Less than 12 months	48,514	61,218
12 months and greater	355,475	391,361
Total Fair Value	403,989	452,579
Amortized Cost
Amortized cost, less than 12 months	49,491	61,697
12 months and greater	444,018	474,496
Total Cost or Amortized Cost	493,509	536,193
Unrealized Loss
Less than 12 months	(977)	(479)
12 months and greater	(88,543)	(83,135)
Total Unrealized Loss	$ (89,520)	$ (83,614)