Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 1,838,086	$ 803,974
Unrealized Loss, Less than Twelve Months	(41,591)	(17,434)
Fair Value, Twelve Months or Longer	9,774,673	9,750,839
Unrealized Loss, Twelve Months or Longer	(1,656,035)	(1,481,923)
Fair Value	11,612,759	10,554,813
Unrealized Loss	(1,697,626)	(1,499,357)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,820,480	767,666
Unrealized Loss, Less than Twelve Months	(40,161)	(14,633)
Fair Value, Twelve Months or Longer	9,430,577	9,423,010
Unrealized Loss, Twelve Months or Longer	(1,582,815)	(1,407,709)
Fair Value	11,251,057	10,190,676
Unrealized Loss	(1,622,976)	(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	2,033	0
Unrealized Loss, Less than Twelve Months	(66)	0
Fair Value, Twelve Months or Longer	360,438	364,006
Unrealized Loss, Twelve Months or Longer	(38,973)	(32,306)
Fair Value	362,471	364,006
Unrealized Loss	(39,039)	(32,306)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	479,111	252,800
Unrealized Loss, Less than Twelve Months	(7,440)	(3,520)
Fair Value, Twelve Months or Longer	1,648,303	1,610,163
Unrealized Loss, Twelve Months or Longer	(447,165)	(399,809)
Fair Value	2,127,414	1,862,963
Unrealized Loss	(454,605)	(403,329)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,469	0
Unrealized Loss, Less than Twelve Months	(5)	0
Fair Value, Twelve Months or Longer	27,862	32,591
Unrealized Loss, Twelve Months or Longer	(10,222)	(10,348)
Fair Value	29,331	32,591
Unrealized Loss	(10,227)	(10,348)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,337,867	514,866
Unrealized Loss, Less than Twelve Months	(32,650)	(11,113)
Fair Value, Twelve Months or Longer	7,322,550	7,345,294
Unrealized Loss, Twelve Months or Longer	(1,083,475)	(961,598)
Fair Value	8,660,417	7,860,160
Unrealized Loss	(1,116,125)	(972,711)
Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	692,151	191,573
Unrealized Loss, Less than Twelve Months	(25,399)	(3,881)
Fair Value, Twelve Months or Longer	4,221,438	4,317,827
Unrealized Loss, Twelve Months or Longer	(613,109)	(530,011)
Fair Value	4,913,589	4,509,400
Unrealized Loss	(638,508)	(533,892)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	361,061	242,099
Unrealized Loss, Less than Twelve Months	(3,813)	(6,584)
Fair Value, Twelve Months or Longer	2,419,494	2,341,424
Unrealized Loss, Twelve Months or Longer	(369,671)	(339,628)
Fair Value	2,780,555	2,583,523
Unrealized Loss	(373,484)	(346,212)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	284,655	81,194
Unrealized Loss, Less than Twelve Months	(3,438)	(648)
Fair Value, Twelve Months or Longer	681,618	686,043
Unrealized Loss, Twelve Months or Longer	(100,695)	(91,959)
Fair Value	966,273	767,237
Unrealized Loss	(104,133)	(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	71,424	70,956
Unrealized Loss, Twelve Months or Longer	(2,980)	(3,648)
Fair Value	71,424	70,956
Unrealized Loss	(2,980)	(3,648)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	17,606	36,308
Unrealized Loss, Less than Twelve Months	(1,430)	(2,801)
Fair Value, Twelve Months or Longer	344,096	327,829
Unrealized Loss, Twelve Months or Longer	(73,220)	(74,214)
Fair Value	361,702	364,137
Unrealized Loss	(74,650)	(77,015)
Below Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	17,606	36,308
Unrealized Loss, Less than Twelve Months	(1,430)	(2,801)
Fair Value, Twelve Months or Longer	333,017	316,541
Unrealized Loss, Twelve Months or Longer	(72,719)	(73,805)
Fair Value	350,623	352,849
Unrealized Loss	(74,149)	(76,606)
Below Investment Grade | Corporates | Industrials
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	0	10,745
Unrealized Loss, Less than Twelve Months	0	(199)
Fair Value, Twelve Months or Longer	146,466	145,697
Unrealized Loss, Twelve Months or Longer	(29,419)	(32,756)
Fair Value	146,466	156,442
Unrealized Loss	(29,419)	(32,955)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	8,704	25,563
Unrealized Loss, Less than Twelve Months	(1,403)	(2,602)
Fair Value, Twelve Months or Longer	166,838	151,190
Unrealized Loss, Twelve Months or Longer	(41,837)	(39,526)
Fair Value	175,542	176,753
Unrealized Loss	(43,240)	(42,128)
Below Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	8,902	0
Unrealized Loss, Less than Twelve Months	(27)	0
Fair Value, Twelve Months or Longer	19,713	19,654
Unrealized Loss, Twelve Months or Longer	(1,463)	(1,523)
Fair Value	28,615	19,654
Unrealized Loss	(1,490)	(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	11,079	11,288
Unrealized Loss, Twelve Months or Longer	(501)	(409)
Fair Value	11,079	11,288
Unrealized Loss	$ (501)	$ (409)