Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 11,829,640	$ 461,943
Unrealized Loss, Less than Twelve Months	(2,202,028)	(9,688)
Fair Value, Twelve Months or Longer	344,271	248,093
Unrealized Loss, Twelve Months or Longer	(148,604)	(24,351)
Fair Value	12,173,911	710,036
Unrealized Loss	(2,350,632)	(34,039)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	11,492,299	445,548
Unrealized Loss, Less than Twelve Months	(2,153,121)	(9,405)
Fair Value, Twelve Months or Longer	266,013	124,933
Unrealized Loss, Twelve Months or Longer	(119,144)	(10,739)
Fair Value	11,758,312	570,481
Unrealized Loss	(2,272,265)	(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	349,350	118
Unrealized Loss, Less than Twelve Months	(35,764)	(1)
Fair Value, Twelve Months or Longer	3,416	3,867
Unrealized Loss, Twelve Months or Longer	(746)	(163)
Fair Value	352,766	3,985
Unrealized Loss	(36,510)	(164)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,728,650	141,310
Unrealized Loss, Less than Twelve Months	(562,887)	(2,824)
Fair Value, Twelve Months or Longer	52,359	2,436
Unrealized Loss, Twelve Months or Longer	(35,589)	(83)
Fair Value	1,781,009	143,746
Unrealized Loss	(598,476)	(2,907)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	5,754	12,567
Unrealized Loss, Less than Twelve Months	(142)	(561)
Fair Value, Twelve Months or Longer	23,983	23,144
Unrealized Loss, Twelve Months or Longer	(10,135)	(4,571)
Fair Value	29,737	35,711
Unrealized Loss	(10,277)	(5,132)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	9,347,013	281,064
Unrealized Loss, Less than Twelve Months	(1,550,151)	(6,003)
Fair Value, Twelve Months or Longer	176,543	95,485
Unrealized Loss, Twelve Months or Longer	(71,963)	(5,922)
Fair Value	9,523,556	376,549
Unrealized Loss	(1,622,114)	(11,925)
Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,895,779	133,654
Unrealized Loss, Less than Twelve Months	(490,451)	(1,507)
Fair Value, Twelve Months or Longer	75,439	52,864
Unrealized Loss, Twelve Months or Longer	(27,653)	(1,932)
Fair Value	2,971,218	186,518
Unrealized Loss	(518,104)	(3,439)
Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,114,356	25,447
Unrealized Loss, Less than Twelve Months	(135,376)	(692)
Fair Value, Twelve Months or Longer	20,829	2,372
Unrealized Loss, Twelve Months or Longer	(6,416)	(320)
Fair Value	1,135,185	27,819
Unrealized Loss	(141,792)	(1,012)
Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	958,397	6,519
Unrealized Loss, Less than Twelve Months	(125,270)	(238)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	958,397	6,519
Unrealized Loss	(125,270)	(238)
Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	4,378,481	115,444
Unrealized Loss, Less than Twelve Months	(799,054)	(3,566)
Fair Value, Twelve Months or Longer	80,275	40,249
Unrealized Loss, Twelve Months or Longer	(37,894)	(3,670)
Fair Value	4,458,756	155,693
Unrealized Loss	(836,948)	(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	61,532	10,489
Unrealized Loss, Less than Twelve Months	(4,177)	(16)
Fair Value, Twelve Months or Longer	9,712	1
Unrealized Loss, Twelve Months or Longer	(711)	0
Fair Value	71,244	10,490
Unrealized Loss	(4,888)	(16)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	337,341	16,395
Unrealized Loss, Less than Twelve Months	(48,907)	(283)
Fair Value, Twelve Months or Longer	78,258	123,160
Unrealized Loss, Twelve Months or Longer	(29,460)	(13,612)
Fair Value	415,599	139,555
Unrealized Loss	(78,367)	(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	337,341	16,395
Unrealized Loss, Less than Twelve Months	(48,907)	(283)
Fair Value, Twelve Months or Longer	66,175	110,117
Unrealized Loss, Twelve Months or Longer	(28,836)	(13,198)
Fair Value	403,516	126,512
Unrealized Loss	(77,743)	(13,481)
Below Investment Grade | Corporates | Financial
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	127,557	15,695
Unrealized Loss, Less than Twelve Months	(18,081)	(272)
Fair Value, Twelve Months or Longer	39,446	56,897
Unrealized Loss, Twelve Months or Longer	(16,193)	(5,638)
Fair Value	167,003	72,592
Unrealized Loss	(34,274)	(5,910)
Below Investment Grade | Corporates | Utilities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	27,925	0
Unrealized Loss, Less than Twelve Months	(2,970)	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	27,925	0
Unrealized Loss	(2,970)	0
Below Investment Grade | Corporates | Energy
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	14,220	0
Unrealized Loss, Less than Twelve Months	(2,453)	0
Fair Value, Twelve Months or Longer	19,868	26,639
Unrealized Loss, Twelve Months or Longer	(8,199)	(1,445)
Fair Value	34,088	26,639
Unrealized Loss	(10,652)	(1,445)
Below Investment Grade | Corporates | Other corporate sectors
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	167,639	700
Unrealized Loss, Less than Twelve Months	(25,403)	(11)
Fair Value, Twelve Months or Longer	6,861	26,581
Unrealized Loss, Twelve Months or Longer	(4,444)	(6,115)
Fair Value	174,500	27,281
Unrealized Loss	(29,847)	(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	12,083	13,043
Unrealized Loss, Twelve Months or Longer	(624)	(414)
Fair Value	12,083	13,043
Unrealized Loss	$ (624)	$ (414)