Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 359,414	$ 803,974
Unrealized Loss, Less than Twelve Months	(16,532)	(17,434)
Fair Value, Twelve Months or Longer	9,191,636	9,750,839
Unrealized Loss, Twelve Months or Longer	(1,321,408)	(1,481,923)
Fair Value	9,551,050	10,554,813
Unrealized Loss	(1,337,940)	(1,499,357)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	352,545	767,666
Unrealized Loss, Less than Twelve Months	(16,350)	(14,633)
Fair Value, Twelve Months or Longer	8,852,284	9,423,010
Unrealized Loss, Twelve Months or Longer	(1,250,268)	(1,407,709)
Fair Value	9,204,829	10,190,676
Unrealized Loss	(1,266,618)	(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	0	0
Unrealized Loss, Less than Twelve Months	0	0
Fair Value, Twelve Months or Longer	374,361	364,006
Unrealized Loss, Twelve Months or Longer	(21,026)	(32,306)
Fair Value	374,361	364,006
Unrealized Loss	(21,026)	(32,306)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	201,640	252,800
Unrealized Loss, Less than Twelve Months	(4,821)	(3,520)
Fair Value, Twelve Months or Longer	1,636,467	1,610,163
Unrealized Loss, Twelve Months or Longer	(420,168)	(399,809)
Fair Value	1,838,107	1,862,963
Unrealized Loss	(424,989)	(403,329)
Investment Grade | Foreign governments
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	28,493	32,591
Unrealized Loss, Twelve Months or Longer	(9,885)	(10,348)
Fair Value	28,493	32,591
Unrealized Loss	(9,885)	(10,348)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	150,905	514,866
Unrealized Loss, Less than Twelve Months	(11,529)	(11,113)
Fair Value, Twelve Months or Longer	6,744,414	7,345,294
Unrealized Loss, Twelve Months or Longer	(797,582)	(961,598)
Fair Value	6,895,319	7,860,160
Unrealized Loss	(809,111)	(972,711)
Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	124,717	191,573
Unrealized Loss, Less than Twelve Months	(11,134)	(3,881)
Fair Value, Twelve Months or Longer	4,071,839	4,317,827
Unrealized Loss, Twelve Months or Longer	(464,628)	(530,011)
Fair Value	4,196,556	4,509,400
Unrealized Loss	(475,762)	(533,892)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	19,176	242,099
Unrealized Loss, Less than Twelve Months	(303)	(6,584)
Fair Value, Twelve Months or Longer	2,005,383	2,341,424
Unrealized Loss, Twelve Months or Longer	(261,465)	(339,628)
Fair Value	2,024,559	2,583,523
Unrealized Loss	(261,768)	(346,212)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	7,012	81,194
Unrealized Loss, Less than Twelve Months	(92)	(648)
Fair Value, Twelve Months or Longer	667,192	686,043
Unrealized Loss, Twelve Months or Longer	(71,489)	(91,959)
Fair Value	674,204	767,237
Unrealized Loss	(71,581)	(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	68,549	70,956
Unrealized Loss, Twelve Months or Longer	(1,607)	(3,648)
Fair Value	68,549	70,956
Unrealized Loss	(1,607)	(3,648)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	6,869	36,308
Unrealized Loss, Less than Twelve Months	(182)	(2,801)
Fair Value, Twelve Months or Longer	339,352	327,829
Unrealized Loss, Twelve Months or Longer	(71,140)	(74,214)
Fair Value	346,221	364,137
Unrealized Loss	(71,322)	(77,015)
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	6,869	36,308
Unrealized Loss, Less than Twelve Months	(182)	(2,801)
Fair Value, Twelve Months or Longer	333,920	316,541
Unrealized Loss, Twelve Months or Longer	(71,070)	(73,805)
Fair Value	340,789	352,849
Unrealized Loss	(71,252)	(76,606)
Below Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,670	10,745
Unrealized Loss, Less than Twelve Months	(169)	(199)
Fair Value, Twelve Months or Longer	143,088	145,697
Unrealized Loss, Twelve Months or Longer	(32,158)	(32,756)
Fair Value	145,758	156,442
Unrealized Loss	(32,327)	(32,955)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	4,199	25,563
Unrealized Loss, Less than Twelve Months	(13)	(2,602)
Fair Value, Twelve Months or Longer	178,887	151,190
Unrealized Loss, Twelve Months or Longer	(38,122)	(39,526)
Fair Value	183,086	176,753
Unrealized Loss	(38,135)	(42,128)
Below Investment Grade | Corporates | Utilities
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	11,945	19,654
Unrealized Loss, Twelve Months or Longer	(790)	(1,523)
Fair Value	11,945	19,654
Unrealized Loss	(790)	(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	5,432	11,288
Unrealized Loss, Twelve Months or Longer	(70)	(409)
Fair Value	5,432	11,288
Unrealized Loss	$ (70)	$ (409)