Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 3,886,032	$ 803,974
Unrealized Loss, Less than Twelve Months	(183,665)	(17,434)
Fair Value, Twelve Months or Longer	8,001,246	9,750,839
Unrealized Loss, Twelve Months or Longer	(1,741,324)	(1,481,923)
Fair Value	11,887,278	10,554,813
Unrealized Loss	(1,924,989)	(1,499,357)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	3,809,580	767,666
Unrealized Loss, Less than Twelve Months	(179,843)	(14,633)
Fair Value, Twelve Months or Longer	7,705,596	9,423,010
Unrealized Loss, Twelve Months or Longer	(1,671,119)	(1,407,709)
Fair Value	11,515,176	10,190,676
Unrealized Loss	(1,850,962)	(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	11,268	0
Unrealized Loss, Less than Twelve Months	(290)	0
Fair Value, Twelve Months or Longer	347,527	364,006
Unrealized Loss, Twelve Months or Longer	(42,504)	(32,306)
Fair Value	358,795	364,006
Unrealized Loss	(42,794)	(32,306)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	778,244	252,800
Unrealized Loss, Less than Twelve Months	(32,894)	(3,520)
Fair Value, Twelve Months or Longer	1,532,264	1,610,163
Unrealized Loss, Twelve Months or Longer	(501,865)	(399,809)
Fair Value	2,310,508	1,862,963
Unrealized Loss	(534,759)	(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	24,925	32,591
Unrealized Loss, Twelve Months or Longer	(8,870)	(10,348)
Fair Value	24,925	32,591
Unrealized Loss	(8,870)	(10,348)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,996,837	514,866
Unrealized Loss, Less than Twelve Months	(146,564)	(11,113)
Fair Value, Twelve Months or Longer	5,758,241	7,345,294
Unrealized Loss, Twelve Months or Longer	(1,115,789)	(961,598)
Fair Value	8,755,078	7,860,160
Unrealized Loss	(1,262,353)	(972,711)
Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,487,940	191,573
Unrealized Loss, Less than Twelve Months	(73,404)	(3,881)
Fair Value, Twelve Months or Longer	3,433,034	4,317,827
Unrealized Loss, Twelve Months or Longer	(690,920)	(530,011)
Fair Value	4,920,974	4,509,400
Unrealized Loss	(764,324)	(533,892)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	961,932	242,099
Unrealized Loss, Less than Twelve Months	(52,946)	(6,584)
Fair Value, Twelve Months or Longer	1,785,130	2,341,424
Unrealized Loss, Twelve Months or Longer	(333,873)	(339,628)
Fair Value	2,747,062	2,583,523
Unrealized Loss	(386,819)	(346,212)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	546,965	81,194
Unrealized Loss, Less than Twelve Months	(20,214)	(648)
Fair Value, Twelve Months or Longer	540,077	686,043
Unrealized Loss, Twelve Months or Longer	(90,996)	(91,959)
Fair Value	1,087,042	767,237
Unrealized Loss	(111,210)	(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	23,231	0
Unrealized Loss, Less than Twelve Months	(95)	0
Fair Value, Twelve Months or Longer	42,639	70,956
Unrealized Loss, Twelve Months or Longer	(2,091)	(3,648)
Fair Value	65,870	70,956
Unrealized Loss	(2,186)	(3,648)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	76,452	36,308
Unrealized Loss, Less than Twelve Months	(3,822)	(2,801)
Fair Value, Twelve Months or Longer	295,650	327,829
Unrealized Loss, Twelve Months or Longer	(70,205)	(74,214)
Fair Value	372,102	364,137
Unrealized Loss	(74,027)	(77,015)
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	76,452	36,308
Unrealized Loss, Less than Twelve Months	(3,822)	(2,801)
Fair Value, Twelve Months or Longer	293,452	316,541
Unrealized Loss, Twelve Months or Longer	(70,205)	(73,805)
Fair Value	369,904	352,849
Unrealized Loss	(74,027)	(76,606)
Below Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	54,199	10,745
Unrealized Loss, Less than Twelve Months	(2,656)	(199)
Fair Value, Twelve Months or Longer	142,638	145,697
Unrealized Loss, Twelve Months or Longer	(38,350)	(32,756)
Fair Value	196,837	156,442
Unrealized Loss	(41,006)	(32,955)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,990	25,563
Unrealized Loss, Less than Twelve Months	(53)	(2,602)
Fair Value, Twelve Months or Longer	126,811	151,190
Unrealized Loss, Twelve Months or Longer	(26,171)	(39,526)
Fair Value	129,801	176,753
Unrealized Loss	(26,224)	(42,128)
Below Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	19,263	0
Unrealized Loss, Less than Twelve Months	(1,113)	0
Fair Value, Twelve Months or Longer	24,003	19,654
Unrealized Loss, Twelve Months or Longer	(5,684)	(1,523)
Fair Value	43,266	19,654
Unrealized Loss	(6,797)	(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	2,198	11,288
Unrealized Loss, Twelve Months or Longer	0	(409)
Fair Value	2,198	11,288
Unrealized Loss	$ 0	$ (409)