Investments - Securities in an Unrealized Loss Position Segregated by Type and Length of Time in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair value
Less than 12 months	$ 434,654	$ 248,510
12 months and greater	1,922,397	1,939,305
Total Fair Value	2,357,051	2,187,815
Amortized Cost
Amortized cost, less than 12 months	441,318	250,863
12 months and greater	2,143,603	2,151,348
Total Cost or Amortized Cost	2,584,921	2,402,211
Unrealized Loss
Less than 12 months	(6,664)	(2,353)
12 months and greater	(221,206)	(212,043)
Total Unrealized Loss	(227,870)	(214,396)
U.S. government
Fair value
Less than 12 months	78,413	37,718
12 months and greater	203,962	204,556
Total Fair Value	282,375	242,274
Amortized Cost
Amortized cost, less than 12 months	79,565	37,950
12 months and greater	210,653	210,182
Total Cost or Amortized Cost	290,218	248,132
Unrealized Loss
Less than 12 months	(1,152)	(232)
12 months and greater	(6,691)	(5,626)
Total Unrealized Loss	(7,843)	(5,858)
U.S. Agency
Fair value
Less than 12 months	15,822	8,736
12 months and greater	28,854	29,632
Total Fair Value	44,676	38,368
Amortized Cost
Amortized cost, less than 12 months	15,995	8,790
12 months and greater	30,626	31,167
Total Cost or Amortized Cost	46,621	39,957
Unrealized Loss
Less than 12 months	(173)	(54)
12 months and greater	(1,772)	(1,535)
Total Unrealized Loss	(1,945)	(1,589)
Non-U.S. government & agency
Fair value
12 months and greater	3,844	3,882
Total Fair Value	3,844	3,882
Amortized Cost
12 months and greater	4,800	4,800
Total Cost or Amortized Cost	4,800	4,800
Unrealized Loss
12 months and greater	(956)	(918)
Total Unrealized Loss	(956)	(918)
Agency MBS
Fair value
Less than 12 months	55,007	61,196
12 months and greater	288,704	275,707
Total Fair Value	343,711	336,903
Amortized Cost
Amortized cost, less than 12 months	56,100	61,714
12 months and greater	330,749	313,091
Total Cost or Amortized Cost	386,849	374,805
Unrealized Loss
Less than 12 months	(1,093)	(518)
12 months and greater	(42,045)	(37,384)
Total Unrealized Loss	(43,138)	(37,902)
ABS/CMBS/MBS
Fair value
Less than 12 months	25,663	12,240
12 months and greater	182,583	211,436
Total Fair Value	208,246	223,676
Amortized Cost
Amortized cost, less than 12 months	25,831	12,367
12 months and greater	209,321	239,193
Total Cost or Amortized Cost	235,152	251,560
Unrealized Loss
Less than 12 months	(168)	(127)
12 months and greater	(26,738)	(27,757)
Total Unrealized Loss	(26,906)	(27,884)
Corporate Debt
Fair value
Less than 12 months	185,542	67,402
12 months and greater	795,631	822,731
Total Fair Value	981,173	890,133
Amortized Cost
Amortized cost, less than 12 months	188,580	68,345
12 months and greater	853,301	878,419
Total Cost or Amortized Cost	1,041,881	946,764
Unrealized Loss
Less than 12 months	(3,038)	(943)
12 months and greater	(57,670)	(55,688)
Total Unrealized Loss	(60,708)	(56,631)
Municipal
Fair value
Less than 12 months	74,207	61,218
12 months and greater	418,819	391,361
Total Fair Value	493,026	452,579
Amortized Cost
Amortized cost, less than 12 months	75,247	61,697
12 months and greater	504,153	474,496
Total Cost or Amortized Cost	579,400	536,193
Unrealized Loss
Less than 12 months	(1,040)	(479)
12 months and greater	(85,334)	(83,135)
Total Unrealized Loss	$ (86,374)	$ (83,614)