Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 803,974	$ 11,450,679
Unrealized Loss, Less than Twelve Months	(17,434)	(1,846,999)
Fair Value, Twelve Months or Longer	9,750,839	433,328
Unrealized Loss, Twelve Months or Longer	(1,481,923)	(190,295)
Fair Value	10,554,813	11,884,007
Unrealized Loss	(1,499,357)	(2,037,294)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	767,666	11,121,941
Unrealized Loss, Less than Twelve Months	(14,633)	(1,796,781)
Fair Value, Twelve Months or Longer	9,423,010	357,361
Unrealized Loss, Twelve Months or Longer	(1,407,709)	(158,773)
Fair Value	10,190,676	11,479,302
Unrealized Loss	(1,422,342)	(1,955,554)
Investment Grade | U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	349,887
Unrealized Loss, Less than Twelve Months	0	(38,218)
Fair Value, Twelve Months or Longer	364,006	3,424
Unrealized Loss, Twelve Months or Longer	(32,306)	(750)
Fair Value	364,006	353,311
Unrealized Loss	(32,306)	(38,968)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	252,800	1,767,624
Unrealized Loss, Less than Twelve Months	(3,520)	(453,149)
Fair Value, Twelve Months or Longer	1,610,163	95,124
Unrealized Loss, Twelve Months or Longer	(399,809)	(52,298)
Fair Value	1,862,963	1,862,748
Unrealized Loss	(403,329)	(505,447)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	6,297
Unrealized Loss, Less than Twelve Months	0	(201)
Fair Value, Twelve Months or Longer	32,591	25,134
Unrealized Loss, Twelve Months or Longer	(10,348)	(12,505)
Fair Value	32,591	31,431
Unrealized Loss	(10,348)	(12,706)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	514,866	8,937,976
Unrealized Loss, Less than Twelve Months	(11,113)	(1,299,990)
Fair Value, Twelve Months or Longer	7,345,294	225,719
Unrealized Loss, Twelve Months or Longer	(961,598)	(90,785)
Fair Value	7,860,160	9,163,695
Unrealized Loss	(972,711)	(1,390,775)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	242,099	2,837,918
Unrealized Loss, Less than Twelve Months	(6,584)	(426,132)
Fair Value, Twelve Months or Longer	2,341,424	109,784
Unrealized Loss, Twelve Months or Longer	(339,628)	(42,173)
Fair Value	2,583,523	2,947,702
Unrealized Loss	(346,212)	(468,305)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	81,194	1,088,219
Unrealized Loss, Less than Twelve Months	(648)	(116,272)
Fair Value, Twelve Months or Longer	686,043	21,636
Unrealized Loss, Twelve Months or Longer	(91,959)	(6,268)
Fair Value	767,237	1,109,855
Unrealized Loss	(92,607)	(122,540)
Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	18,301	855,853
Unrealized Loss, Less than Twelve Months	(445)	(91,755)
Fair Value, Twelve Months or Longer	516,387	0
Unrealized Loss, Twelve Months or Longer	(54,398)	0
Fair Value	534,688	855,853
Unrealized Loss	(54,843)	(91,755)
Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	173,272	4,155,986
Unrealized Loss, Less than Twelve Months	(3,436)	(665,831)
Fair Value, Twelve Months or Longer	3,801,440	94,299
Unrealized Loss, Twelve Months or Longer	(475,613)	(42,344)
Fair Value	3,974,712	4,250,285
Unrealized Loss	(479,049)	(708,175)
Investment Grade | Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	0
Unrealized Loss	0	0
Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	60,157
Unrealized Loss, Less than Twelve Months	0	(5,223)
Fair Value, Twelve Months or Longer	70,956	7,960
Unrealized Loss, Twelve Months or Longer	(3,648)	(2,435)
Fair Value	70,956	68,117
Unrealized Loss	(3,648)	(7,658)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	36,308	328,738
Unrealized Loss, Less than Twelve Months	(2,801)	(50,218)
Fair Value, Twelve Months or Longer	327,829	75,967
Unrealized Loss, Twelve Months or Longer	(74,214)	(31,522)
Fair Value	364,137	404,705
Unrealized Loss	(77,015)	(81,740)
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	36,308	328,738
Unrealized Loss, Less than Twelve Months	(2,801)	(50,218)
Fair Value, Twelve Months or Longer	316,541	65,093
Unrealized Loss, Twelve Months or Longer	(73,805)	(29,904)
Fair Value	352,849	393,831
Unrealized Loss	(76,606)	(80,122)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	25,563	120,377
Unrealized Loss, Less than Twelve Months	(2,602)	(18,901)
Fair Value, Twelve Months or Longer	151,190	38,348
Unrealized Loss, Twelve Months or Longer	(39,526)	(17,283)
Fair Value	176,753	158,725
Unrealized Loss	(42,128)	(36,184)
Below Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	27,722
Unrealized Loss, Less than Twelve Months	0	(3,173)
Fair Value, Twelve Months or Longer	19,654	0
Unrealized Loss, Twelve Months or Longer	(1,523)	0
Fair Value	19,654	27,722
Unrealized Loss	(1,523)	(3,173)
Below Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	14,480
Unrealized Loss, Less than Twelve Months	0	(2,182)
Fair Value, Twelve Months or Longer	37,171	20,075
Unrealized Loss, Twelve Months or Longer	(7,481)	(7,986)
Fair Value	37,171	34,555
Unrealized Loss	(7,481)	(10,168)
Below Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	10,745	166,159
Unrealized Loss, Less than Twelve Months	(199)	(25,962)
Fair Value, Twelve Months or Longer	108,526	6,670
Unrealized Loss, Twelve Months or Longer	(25,275)	(4,635)
Fair Value	119,271	172,829
Unrealized Loss	(25,474)	(30,597)
Below Investment Grade | Collateralized debt obligations
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	0	0
Unrealized Loss	0	0
Below Investment Grade | Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	11,288	10,874
Unrealized Loss, Twelve Months or Longer	(409)	(1,618)
Fair Value	11,288	10,874
Unrealized Loss	$ (409)	$ (1,618)