Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 2,600,310	$ 803,974
Unrealized Loss, Less than Twelve Months	(73,916)	(17,434)
Fair Value, Twelve Months or Longer	9,728,309	9,750,839
Unrealized Loss, Twelve Months or Longer	(1,799,554)	(1,481,923)
Fair Value	12,328,619	10,554,813
Unrealized Loss	(1,873,470)	(1,499,357)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,527,158	767,666
Unrealized Loss, Less than Twelve Months	(71,753)	(14,633)
Fair Value, Twelve Months or Longer	9,368,890	9,423,010
Unrealized Loss, Twelve Months or Longer	(1,724,683)	(1,407,709)
Fair Value	11,896,048	10,190,676
Unrealized Loss	(1,796,436)	(1,422,342)
Investment Grade | U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,304	0
Unrealized Loss, Less than Twelve Months	(110)	0
Fair Value, Twelve Months or Longer	358,980	364,006
Unrealized Loss, Twelve Months or Longer	(41,776)	(32,306)
Fair Value	361,284	364,006
Unrealized Loss	(41,886)	(32,306)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	518,622	252,800
Unrealized Loss, Less than Twelve Months	(13,082)	(3,520)
Fair Value, Twelve Months or Longer	1,786,640	1,610,163
Unrealized Loss, Twelve Months or Longer	(460,274)	(399,809)
Fair Value	2,305,262	1,862,963
Unrealized Loss	(473,356)	(403,329)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,465	0
Unrealized Loss, Less than Twelve Months	(2)	0
Fair Value, Twelve Months or Longer	28,483	32,591
Unrealized Loss, Twelve Months or Longer	(10,535)	(10,348)
Fair Value	29,948	32,591
Unrealized Loss	(10,537)	(10,348)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,004,767	514,866
Unrealized Loss, Less than Twelve Months	(58,559)	(11,113)
Fair Value, Twelve Months or Longer	7,123,913	7,345,294
Unrealized Loss, Twelve Months or Longer	(1,208,866)	(961,598)
Fair Value	9,128,680	7,860,160
Unrealized Loss	(1,267,425)	(972,711)
Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	930,937	191,573
Unrealized Loss, Less than Twelve Months	(38,817)	(3,881)
Fair Value, Twelve Months or Longer	4,175,164	4,317,827
Unrealized Loss, Twelve Months or Longer	(708,164)	(530,011)
Fair Value	5,106,101	4,509,400
Unrealized Loss	(746,981)	(533,892)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	617,484	242,099
Unrealized Loss, Less than Twelve Months	(11,013)	(6,584)
Fair Value, Twelve Months or Longer	2,230,054	2,341,424
Unrealized Loss, Twelve Months or Longer	(386,505)	(339,628)
Fair Value	2,847,538	2,583,523
Unrealized Loss	(397,518)	(346,212)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	456,346	81,194
Unrealized Loss, Less than Twelve Months	(8,729)	(648)
Fair Value, Twelve Months or Longer	718,695	686,043
Unrealized Loss, Twelve Months or Longer	(114,197)	(91,959)
Fair Value	1,175,041	767,237
Unrealized Loss	(122,926)	(92,607)
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	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	70,874	70,956
Unrealized Loss, Twelve Months or Longer	(3,232)	(3,648)
Fair Value	70,874	70,956
Unrealized Loss	(3,232)	(3,648)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	73,152	36,308
Unrealized Loss, Less than Twelve Months	(2,163)	(2,801)
Fair Value, Twelve Months or Longer	359,419	327,829
Unrealized Loss, Twelve Months or Longer	(74,871)	(74,214)
Fair Value	432,571	364,137
Unrealized Loss	(77,034)	(77,015)
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	73,152	36,308
Unrealized Loss, Less than Twelve Months	(2,163)	(2,801)
Fair Value, Twelve Months or Longer	353,447	316,541
Unrealized Loss, Twelve Months or Longer	(74,854)	(73,805)
Fair Value	426,599	352,849
Unrealized Loss	(77,017)	(76,606)
Below Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	55,327	10,745
Unrealized Loss, Less than Twelve Months	(953)	(199)
Fair Value, Twelve Months or Longer	142,449	145,697
Unrealized Loss, Twelve Months or Longer	(32,877)	(32,756)
Fair Value	197,776	156,442
Unrealized Loss	(33,830)	(32,955)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	8,909	25,563
Unrealized Loss, Less than Twelve Months	(1,196)	(2,602)
Fair Value, Twelve Months or Longer	192,106	151,190
Unrealized Loss, Twelve Months or Longer	(40,491)	(39,526)
Fair Value	201,015	176,753
Unrealized Loss	(41,687)	(42,128)
Below Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	8,916	0
Unrealized Loss, Less than Twelve Months	(14)	0
Fair Value, Twelve Months or Longer	18,892	19,654
Unrealized Loss, Twelve Months or Longer	(1,486)	(1,523)
Fair Value	27,808	19,654
Unrealized Loss	(1,500)	(1,523)
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	5,972	11,288
Unrealized Loss, Twelve Months or Longer	(17)	(409)
Fair Value	5,972	11,288
Unrealized Loss	$ (17)	$ (409)