Investments - Schedule of Unrealized Investment Losses by Class of Investment (Detail) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	$ 9,800,863	$ 461,943
Unrealized Loss, Less than Twelve Months	(1,283,837)	(9,688)
Fair Value, Twelve Months or Longer	221,000	248,093
Unrealized Loss, Twelve Months or Longer	(75,633)	(24,351)
Fair Value	10,021,863	710,036
Unrealized Loss	(1,359,470)	(34,039)
Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	9,481,333	445,548
Unrealized Loss, Less than Twelve Months	(1,243,571)	(9,405)
Fair Value, Twelve Months or Longer	139,227	124,933
Unrealized Loss, Twelve Months or Longer	(49,235)	(10,739)
Fair Value	9,620,560	570,481
Unrealized Loss	(1,292,806)	(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	289,383	118
Unrealized Loss, Less than Twelve Months	(7,858)	(1)
Fair Value, Twelve Months or Longer	3,465	3,867
Unrealized Loss, Twelve Months or Longer	(580)	(163)
Fair Value	292,848	3,985
Unrealized Loss	(8,438)	(164)
Investment Grade | States, municipalities, and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	1,674,505	141,310
Unrealized Loss, Less than Twelve Months	(386,550)	(2,824)
Fair Value, Twelve Months or Longer	4,066	2,436
Unrealized Loss, Twelve Months or Longer	(2,081)	(83)
Fair Value	1,678,571	143,746
Unrealized Loss	(388,631)	(2,907)
Investment Grade | Foreign governments
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	9,838	12,567
Unrealized Loss, Less than Twelve Months	(1,254)	(561)
Fair Value, Twelve Months or Longer	23,380	23,144
Unrealized Loss, Twelve Months or Longer	(8,429)	(4,571)
Fair Value	33,218	35,711
Unrealized Loss	(9,683)	(5,132)
Investment Grade | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	7,433,029	281,064
Unrealized Loss, Less than Twelve Months	(846,096)	(6,003)
Fair Value, Twelve Months or Longer	108,316	95,485
Unrealized Loss, Twelve Months or Longer	(38,145)	(5,922)
Fair Value	7,541,345	376,549
Unrealized Loss	(884,241)	(11,925)
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	74,578	10,489
Unrealized Loss, Less than Twelve Months	(1,813)	(16)
Fair Value, Twelve Months or Longer	0	1
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	74,578	10,490
Unrealized Loss	(1,813)	(16)
Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	2,260,544	133,654
Unrealized Loss, Less than Twelve Months	(284,797)	(1,507)
Fair Value, Twelve Months or Longer	50,713	52,864
Unrealized Loss, Twelve Months or Longer	(14,674)	(1,932)
Fair Value	2,311,257	186,518
Unrealized Loss	(299,471)	(3,439)
Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	650,822	25,447
Unrealized Loss, Less than Twelve Months	(65,538)	(692)
Fair Value, Twelve Months or Longer	3,970	2,372
Unrealized Loss, Twelve Months or Longer	(1,530)	(320)
Fair Value	654,792	27,819
Unrealized Loss	(67,068)	(1,012)
Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	662,038	6,519
Unrealized Loss, Less than Twelve Months	(57,153)	(238)
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	662,038	6,519
Unrealized Loss	(57,153)	(238)
Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	3,859,625	115,444
Unrealized Loss, Less than Twelve Months	(438,608)	(3,566)
Fair Value, Twelve Months or Longer	53,633	40,249
Unrealized Loss, Twelve Months or Longer	(21,941)	(3,670)
Fair Value	3,913,258	155,693
Unrealized Loss	(460,549)	(7,236)
Below Investment Grade
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	319,530	16,395
Unrealized Loss, Less than Twelve Months	(40,266)	(283)
Fair Value, Twelve Months or Longer	81,773	123,160
Unrealized Loss, Twelve Months or Longer	(26,398)	(13,612)
Fair Value	401,303	139,555
Unrealized Loss	(66,664)	(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	319,530	16,395
Unrealized Loss, Less than Twelve Months	(40,266)	(283)
Fair Value, Twelve Months or Longer	69,280	110,117
Unrealized Loss, Twelve Months or Longer	(25,781)	(13,198)
Fair Value	388,810	126,512
Unrealized Loss	(66,047)	(13,481)
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	12,493	13,043
Unrealized Loss, Twelve Months or Longer	(617)	(414)
Fair Value	12,493	13,043
Unrealized Loss	(617)	(414)
Below Investment Grade | Financial | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	79,869	15,695
Unrealized Loss, Less than Twelve Months	(10,269)	(272)
Fair Value, Twelve Months or Longer	43,105	56,897
Unrealized Loss, Twelve Months or Longer	(12,543)	(5,638)
Fair Value	122,974	72,592
Unrealized Loss	(22,812)	(5,910)
Below Investment Grade | Utilities | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	28,547	0
Unrealized Loss, Less than Twelve Months	(2,350)	0
Fair Value, Twelve Months or Longer	0	0
Unrealized Loss, Twelve Months or Longer	0	0
Fair Value	28,547	0
Unrealized Loss	(2,350)	0
Below Investment Grade | Energy | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	48,666	0
Unrealized Loss, Less than Twelve Months	(8,428)	0
Fair Value, Twelve Months or Longer	19,176	26,639
Unrealized Loss, Twelve Months or Longer	(8,896)	(1,445)
Fair Value	67,842	26,639
Unrealized Loss	(17,324)	(1,445)
Below Investment Grade | Other corporate sectors | Corporates
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than Twelve Months	162,448	700
Unrealized Loss, Less than Twelve Months	(19,219)	(11)
Fair Value, Twelve Months or Longer	6,999	26,581
Unrealized Loss, Twelve Months or Longer	(4,342)	(6,115)
Fair Value	169,447	27,281
Unrealized Loss	$ (23,561)	$ (6,126)