Investments - Schedule of Unrealized Investment Losses by Class of Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 11,450,679	$ 461,943
Unrealized Loss, Less than Twelve Months	(1,846,999)	(9,688)
Fair Value, Twelve Months or Longer	433,328	248,093
Unrealized Loss, Twelve Months or Longer	(190,295)	(24,351)
Fair Value	11,884,007	710,036
Unrealized Loss	(2,037,294)	(34,039)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	11,121,941	445,548
Unrealized Loss, Less than Twelve Months	(1,796,781)	(9,405)
Fair Value, Twelve Months or Longer	357,361	124,933
Unrealized Loss, Twelve Months or Longer	(158,773)	(10,739)
Fair Value	11,479,302	570,481
Unrealized Loss	(1,955,554)	(20,144)
Investment Grade | U.S. Government direct, guaranteed, and government-sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	349,887	118
Unrealized Loss, Less than Twelve Months	(38,218)	(1)
Fair Value, Twelve Months or Longer	3,424	3,867
Unrealized Loss, Twelve Months or Longer	(750)	(163)
Fair Value	353,311	3,985
Unrealized Loss	(38,968)	(164)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,767,624	141,310
Unrealized Loss, Less than Twelve Months	(453,149)	(2,824)
Fair Value, Twelve Months or Longer	95,124	2,436
Unrealized Loss, Twelve Months or Longer	(52,298)	(83)
Fair Value	1,862,748	143,746
Unrealized Loss	(505,447)	(2,907)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	6,297	12,567
Unrealized Loss, Less than Twelve Months	(201)	(561)
Fair Value, Twelve Months or Longer	25,134	23,144
Unrealized Loss, Twelve Months or Longer	(12,505)	(4,571)
Fair Value	31,431	35,711
Unrealized Loss	(12,706)	(5,132)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	8,937,976	281,064
Unrealized Loss, Less than Twelve Months	(1,299,990)	(6,003)
Fair Value, Twelve Months or Longer	225,719	95,485
Unrealized Loss, Twelve Months or Longer	(90,785)	(5,922)
Fair Value	9,163,695	376,549
Unrealized Loss	(1,390,775)	(11,925)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,837,918	133,654
Unrealized Loss, Less than Twelve Months	(426,132)	(1,507)
Fair Value, Twelve Months or Longer	109,784	52,864
Unrealized Loss, Twelve Months or Longer	(42,173)	(1,932)
Fair Value	2,947,702	186,518
Unrealized Loss	(468,305)	(3,439)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,088,219	25,447
Unrealized Loss, Less than Twelve Months	(116,272)	(692)
Fair Value, Twelve Months or Longer	21,636	2,372
Unrealized Loss, Twelve Months or Longer	(6,268)	(320)
Fair Value	1,109,855	27,819
Unrealized Loss	(122,540)	(1,012)
Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	855,853	6,519
Unrealized Loss, Less than Twelve Months	(91,755)	(238)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	855,853	6,519
Unrealized Loss	(91,755)	(238)
Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	4,155,986	115,444
Unrealized Loss, Less than Twelve Months	(665,831)	(3,566)
Fair Value, Twelve Months or Longer	94,299	40,249
Unrealized Loss, Twelve Months or Longer	(42,344)	(3,670)
Fair Value	4,250,285	155,693
Unrealized Loss	(708,175)	(7,236)
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	60,157	10,489
Unrealized Loss, Less than Twelve Months	(5,223)	(16)
Fair Value, Twelve Months or Longer	7,960	1
Unrealized Loss, Twelve Months or Longer	(2,435)	0
Fair Value	68,117	10,490
Unrealized Loss	(7,658)	(16)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	328,738	16,395
Unrealized Loss, Less than Twelve Months	(50,218)	(283)
Fair Value, Twelve Months or Longer	75,967	123,160
Unrealized Loss, Twelve Months or Longer	(31,522)	(13,612)
Fair Value	404,705	139,555
Unrealized Loss	(81,740)	(13,895)
Below Investment Grade | States, municipalities, and political subdivisions
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 | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	328,738	16,395
Unrealized Loss, Less than Twelve Months	(50,218)	(283)
Fair Value, Twelve Months or Longer	65,093	110,117
Unrealized Loss, Twelve Months or Longer	(29,904)	(13,198)
Fair Value	393,831	126,512
Unrealized Loss	(80,122)	(13,481)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	120,377	15,695
Unrealized Loss, Less than Twelve Months	(18,901)	(272)
Fair Value, Twelve Months or Longer	38,348	56,897
Unrealized Loss, Twelve Months or Longer	(17,283)	(5,638)
Fair Value	158,725	72,592
Unrealized Loss	(36,184)	(5,910)
Below Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	27,722	0
Unrealized Loss, Less than Twelve Months	(3,173)	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	27,722	0
Unrealized Loss	(3,173)	0
Below Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	14,480	0
Unrealized Loss, Less than Twelve Months	(2,182)	0
Fair Value, Twelve Months or Longer	20,075	26,639
Unrealized Loss, Twelve Months or Longer	(7,986)	(1,445)
Fair Value	34,555	26,639
Unrealized Loss	(10,168)	(1,445)
Below Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	166,159	700
Unrealized Loss, Less than Twelve Months	(25,962)	(11)
Fair Value, Twelve Months or Longer	6,670	26,581
Unrealized Loss, Twelve Months or Longer	(4,635)	(6,115)
Fair Value	172,829	27,281
Unrealized Loss	(30,597)	(6,126)
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	10,874	13,043
Unrealized Loss, Twelve Months or Longer	(1,618)	(414)
Fair Value	10,874	13,043
Unrealized Loss	$ (1,618)	$ (414)